Core Java Volume 1: Fundamentals, 11th Edition, Cay S. Horstmann

From the Library of Hristo Dimov Hristov Core Java Volume I–Fundamentals Eleventh Edition From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov Core Java Volume I–Fundamentals Eleventh Edition Cay S. Horstmann Boston • Columbus • New York • San Francisco • Amsterdam • Cape Town Dubai • London • Madrid • Milan • Munich • Paris • Montreal • Toronto • Delhi • Mexico City São Paulo • Sydney • Hong Kong • Seoul • Singapore • Taipei • Tokyo From the Library of Hristo Dimov Hristov The author and publisher have taken care in the preparation of this book, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information or programs contained herein. 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ISBN-13: 978-0-13-516630-7 ISBN-10: 0-13-516630-6 ScoutAutomatedPrintCode From the Library of Hristo Dimov Hristov Contents Preface .......................................................................................................... xix Acknowledgments ......................................................................................... xxv Chapter 1: An Introduction to Java ............................................................... 1 1.1 1.2 1.3 1.4 1.5 Java as a Programming Platform ......................................................... 1 The Java “White Paper” Buzzwords .................................................... 2 1.2.1 Simple ....................................................................................... 3 1.2.2 Object-Oriented ....................................................................... 4 1.2.3 Distributed ............................................................................... 4 1.2.4 Robust ....................................................................................... 4 1.2.5 Secure ....................................................................................... 5 1.2.6 Architecture-Neutral ................................................................ 6 1.2.7 Portable .................................................................................... 6 1.2.8 Interpreted ................................................................................ 7 1.2.9 High-Performance ................................................................... 7 1.2.10 Multithreaded .......................................................................... 8 1.2.11 Dynamic ................................................................................... 8 Java Applets and the Internet ............................................................. 9 A Short History of Java ...................................................................... 10 Common Misconceptions about Java ............................................... 13 Chapter 2: The Java Programming Environment ....................................... 17 2.1 2.2 2.3 2.4 Installing the Java Development Kit ................................................. 2.1.1 Downloading the JDK ........................................................... 2.1.2 Setting up the JDK ................................................................ 2.1.3 Installing Source Files and Documentation ........................ Using the Command-Line Tools ....................................................... Using an Integrated Development Environment ............................. JShell .................................................................................................... 18 18 20 22 23 29 32 v From the Library of Hristo Dimov Hristov vi Contents Chapter 3: Fundamental Programming Structures in Java ...................... 37 3.1 3.2 3.3 3.4 3.5 3.6 3.7 A Simple Java Program ...................................................................... Comments ............................................................................................ Data Types ........................................................................................... 3.3.1 Integer Types ......................................................................... 3.3.2 Floating-Point Types ............................................................. 3.3.3 The char Type .......................................................................... 3.3.4 Unicode and the char Type ................................................... 3.3.5 The boolean Type ..................................................................... Variables and Constants .................................................................... 3.4.1 Declaring Variables ............................................................... 3.4.2 Initializing Variables ............................................................. 3.4.3 Constants ................................................................................ 3.4.4 Enumerated Types ................................................................. Operators ............................................................................................. 3.5.1 Arithmetic Operators ............................................................ 3.5.2 Mathematical Functions and Constants .............................. 3.5.3 Conversions between Numeric Types ................................ 3.5.4 Casts ........................................................................................ 3.5.5 Combining Assignment with Operators ............................. 3.5.6 Increment and Decrement Operators ................................. 3.5.7 Relational and boolean Operators .......................................... 3.5.8 Bitwise Operators .................................................................. 3.5.9 Parentheses and Operator Hierarchy .................................. Strings .................................................................................................. 3.6.1 Substrings ............................................................................... 3.6.2 Concatenation ........................................................................ 3.6.3 Strings Are Immutable .......................................................... 3.6.4 Testing Strings for Equality .................................................. 3.6.5 Empty and Null Strings ......................................................... 3.6.6 Code Points and Code Units ............................................... 3.6.7 The String API ......................................................................... 3.6.8 Reading the Online API Documentation ............................ 3.6.9 Building Strings ..................................................................... Input and Output ................................................................................ 38 41 42 43 44 46 47 48 48 48 50 51 52 52 52 54 56 57 58 58 59 60 61 62 62 63 63 65 66 66 68 71 74 75 From the Library of Hristo Dimov Hristov Contents vii 3.7.1 Reading Input ........................................................................ 75 3.7.2 Formatting Output ................................................................. 78 3.7.3 File Input and Output ........................................................... 83 3.8 Control Flow ........................................................................................ 86 3.8.1 Block Scope ............................................................................ 86 3.8.2 Conditional Statements ......................................................... 87 3.8.3 Loops ...................................................................................... 91 3.8.4 Determinate Loops ................................................................ 95 3.8.5 Multiple Selections The switch Statement ............................ 99 3.8.6 Statements That Break Control Flow ................................ 102 3.9 Big Numbers ...................................................................................... 105 3.10 Arrays ................................................................................................. 108 3.10.1 Declaring Arrays .................................................................. 108 3.10.2 Accessing Array Elements .................................................. 109 3.10.3 The “for each” Loop ............................................................ 110 3.10.4 Array Copying ...................................................................... 111 3.10.5 Command-Line Parameters ................................................ 112 3.10.6 Array Sorting ........................................................................ 113 3.10.7 Multidimensional Arrays .................................................... 116 3.10.8 Ragged Arrays ...................................................................... 120 Chapter 4: Objects and Classes ................................................................ 125 4.1 4.2 4.3 Introduction to Object-Oriented Programming ............................. 4.1.1 Classes .................................................................................. 4.1.2 Objects .................................................................................. 4.1.3 Identifying Classes ............................................................... 4.1.4 Relationships between Classes .......................................... Using Predefined Classes ................................................................. 4.2.1 Objects and Object Variables ............................................. 4.2.2 The LocalDate Class of the Java Library ............................... 4.2.3 Mutator and Accessor Methods ......................................... Defining Your Own Classes ............................................................ 4.3.1 An Employee Class ................................................................... 4.3.2 Use of Multiple Source Files .............................................. 4.3.3 Dissecting the Employee Class ................................................ 4.3.4 First Steps with Constructors ............................................. 126 127 128 129 129 131 132 135 138 141 142 145 146 146 From the Library of Hristo Dimov Hristov viii Contents 4.4 4.5 4.6 4.7 4.8 4.3.5 Declaring Local Variables with var ..................................... 4.3.6 Working with null References ............................................. 4.3.7 Implicit and Explicit Parameters ........................................ 4.3.8 Benefits of Encapsulation .................................................... 4.3.9 Class-Based Access Privileges ............................................ 4.3.10 Private Methods ................................................................... 4.3.11 Final Instance Fields ............................................................ Static Fields and Methods ................................................................ 4.4.1 Static Fields .......................................................................... 4.4.2 Static Constants ................................................................... 4.4.3 Static Methods ..................................................................... 4.4.4 Factory Methods .................................................................. 4.4.5 The main Method ................................................................... Method Parameters ........................................................................... Object Construction .......................................................................... 4.6.1 Overloading .......................................................................... 4.6.2 Default Field Initialization .................................................. 4.6.3 The Constructor with No Arguments ................................ 4.6.4 Explicit Field Initialization .................................................. 4.6.5 Parameter Names ................................................................ 4.6.6 Calling Another Constructor .............................................. 4.6.7 Initialization Blocks ............................................................. 4.6.8 Object Destruction and the finalize Method ..................... Packages ............................................................................................. 4.7.1 Package Names .................................................................... 4.7.2 Class Importation ................................................................ 4.7.3 Static Imports ....................................................................... 4.7.4 Addition of a Class into a Package ................................... 4.7.5 Package Access .................................................................... 4.7.6 The Class Path ..................................................................... 4.7.7 Setting the Class Path ......................................................... JAR Files ............................................................................................. 4.8.1 Creating JAR files ................................................................ 4.8.2 The Manifest ........................................................................ 4.8.3 Executable JAR Files ............................................................ 148 148 150 151 154 155 155 156 156 157 158 159 160 163 170 170 171 172 173 174 175 175 180 180 181 181 183 184 187 189 191 192 192 193 194 From the Library of Hristo Dimov Hristov Contents 4.8.4 Multi-Release JAR Files ....................................................... 4.8.5 A Note about Command-Line Options ............................. 4.9 Documentation Comments .............................................................. 4.9.1 Comment Insertion ............................................................. 4.9.2 Class Comments .................................................................. 4.9.3 Method Comments .............................................................. 4.9.4 Field Comments ................................................................... 4.9.5 General Comments .............................................................. 4.9.6 Package Comments ............................................................. 4.9.7 Comment Extraction ........................................................... 4.10 Class Design Hints ............................................................................ ix 195 197 198 199 199 200 201 201 202 203 204 Chapter 5: Inheritance ................................................................................ 207 5.1 5.2 5.3 5.4 5.5 Classes, Superclasses, and Subclasses ............................................ 5.1.1 Defining Subclasses ............................................................. 5.1.2 Overriding Methods ............................................................ 5.1.3 Subclass Constructors ......................................................... 5.1.4 Inheritance Hierarchies ....................................................... 5.1.5 Polymorphism ...................................................................... 5.1.6 Understanding Method Calls ............................................. 5.1.7 Preventing Inheritance: Final Classes and Methods ....... 5.1.8 Casting .................................................................................. 5.1.9 Abstract Classes ................................................................... 5.1.10 Protected Access .................................................................. Object: The Cosmic Superclass .......................................................... 5.2.1 Variables of Type Object ....................................................... 5.2.2 The equals Method ................................................................ 5.2.3 Equality Testing and Inheritance ....................................... 5.2.4 The hashCode Method ............................................................. 5.2.5 The toString Method ............................................................. Generic Array Lists ........................................................................... 5.3.1 Declaring Array Lists ........................................................... 5.3.2 Accessing Array List Elements ........................................... 5.3.3 Compatibility between Typed and Raw Array Lists ........ Object Wrappers and Autoboxing .................................................. Methods with a Variable Number of Parameters .......................... 208 208 210 211 216 217 218 221 223 225 231 232 232 233 234 238 241 248 248 251 255 256 260 From the Library of Hristo Dimov Hristov x Contents 5.6 5.7 5.8 Enumeration Classes ........................................................................ Reflection ........................................................................................... 5.7.1 The Class Class ...................................................................... 5.7.2 A Primer on Declaring Exceptions .................................... 5.7.3 Resources .............................................................................. 5.7.4 Using Reflection to Analyze the Capabilities of Classes 5.7.5 Using Reflection to Analyze Objects at Runtime ............ 5.7.6 Using Reflection to Write Generic Array Code ............... 5.7.7 Invoking Arbitrary Methods and Constructors ................ Design Hints for Inheritance ........................................................... 261 264 264 267 268 271 277 283 286 290 Chapter 6: Interfaces, Lambda Expressions, and Inner Classes ........... 295 6.1 6.2 6.3 Interfaces ............................................................................................ 6.1.1 The Interface Concept ......................................................... 6.1.2 Properties of Interfaces ....................................................... 6.1.3 Interfaces and Abstract Classes ......................................... 6.1.4 Static and Private Methods ................................................. 6.1.5 Default Methods .................................................................. 6.1.6 Resolving Default Method Conflicts ................................. 6.1.7 Interfaces and Callbacks ..................................................... 6.1.8 The Comparator Interface ......................................................... 6.1.9 Object Cloning ..................................................................... Lambda Expressions ......................................................................... 6.2.1 Why Lambdas? ..................................................................... 6.2.2 The Syntax of Lambda Expressions ................................... 6.2.3 Functional Interfaces ........................................................... 6.2.4 Method References .............................................................. 6.2.5 Constructor References ....................................................... 6.2.6 Variable Scope ..................................................................... 6.2.7 Processing Lambda Expressions ........................................ 6.2.8 More about Comparators ................................................... Inner Classes ..................................................................................... 6.3.1 Use of an Inner Class to Access Object State .................. 6.3.2 Special Syntax Rules for Inner Classes ............................. 6.3.3 Are Inner Classes Useful? Actually Necessary? Secure? 6.3.4 Local Inner Classes .............................................................. 296 296 303 305 306 307 308 310 313 314 322 322 323 326 328 332 333 335 339 340 341 345 346 349 From the Library of Hristo Dimov Hristov Contents 6.4 6.5 6.3.5 Accessing Variables from Outer Methods ........................ 6.3.6 Anonymous Inner Classes .................................................. 6.3.7 Static Inner Classes ............................................................. Service Loaders ................................................................................. Proxies ................................................................................................ 6.5.1 When to Use Proxies .......................................................... 6.5.2 Creating Proxy Objects ....................................................... 6.5.3 Properties of Proxy Classes ................................................ xi 350 352 356 360 362 363 363 368 Chapter 7: Exceptions, Assertions, and Logging .................................... 371 7.1 7.2 7.3 7.4 7.5 Dealing with Errors ........................................................................... 7.1.1 The Classification of Exceptions ........................................ 7.1.2 Declaring Checked Exceptions ........................................... 7.1.3 How to Throw an Exception .............................................. 7.1.4 Creating Exception Classes ................................................ Catching Exceptions ......................................................................... 7.2.1 Catching an Exception ........................................................ 7.2.2 Catching Multiple Exceptions ............................................ 7.2.3 Rethrowing and Chaining Exceptions ............................... 7.2.4 The finally Clause ................................................................ 7.2.5 The try-with-Resources Statement ..................................... 7.2.6 Analyzing Stack Trace Elements ........................................ Tips for Using Exceptions ................................................................ Using Assertions ............................................................................... 7.4.1 The Assertion Concept ....................................................... 7.4.2 Assertion Enabling and Disabling ..................................... 7.4.3 Using Assertions for Parameter Checking ........................ 7.4.4 Using Assertions for Documenting Assumptions ............ Logging ............................................................................................... 7.5.1 Basic Logging ....................................................................... 7.5.2 Advanced Logging ............................................................... 7.5.3 Changing the Log Manager Configuration ....................... 7.5.4 Localization .......................................................................... 7.5.5 Handlers ............................................................................... 7.5.6 Filters .................................................................................... 7.5.7 Formatters ............................................................................. 372 373 375 378 380 381 381 383 384 386 389 391 396 399 399 400 401 402 403 404 405 407 409 410 414 415 From the Library of Hristo Dimov Hristov xii Contents 7.6 7.5.8 A Logging Recipe ................................................................ 415 Debugging Tips ................................................................................. 425 Chapter 8: Generic Programming ............................................................. 431 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 Why Generic Programming? ............................................................ 8.1.1 The Advantage of Type Parameters .................................. 8.1.2 Who Wants to Be a Generic Programmer? ...................... Defining a Simple Generic Class .................................................... Generic Methods ............................................................................... Bounds for Type Variables ............................................................... Generic Code and the Virtual Machine ......................................... 8.5.1 Type Erasure ........................................................................ 8.5.2 Translating Generic Expressions ........................................ 8.5.3 Translating Generic Methods ............................................. 8.5.4 Calling Legacy Code ........................................................... Restrictions and Limitations ............................................................ 8.6.1 Type Parameters Cannot Be Instantiated with Primitive Types ..................................................................................... 8.6.2 Runtime Type Inquiry Only Works with Raw Types ...... 8.6.3 You Cannot Create Arrays of Parameterized Types ...... 8.6.4 Varargs Warnings ................................................................ 8.6.5 You Cannot Instantiate Type Variables ............................ 8.6.6 You Cannot Construct a Generic Array ............................ 8.6.7 Type Variables Are Not Valid in Static Contexts of Generic Classes .................................................................... 8.6.8 You Cannot Throw or Catch Instances of a Generic Class ...................................................................................... 8.6.9 You Can Defeat Checked Exception Checking ................ 8.6.10 Beware of Clashes after Erasure ........................................ Inheritance Rules for Generic Types .............................................. Wildcard Types ................................................................................. 8.8.1 The Wildcard Concept ........................................................ 8.8.2 Supertype Bounds for Wildcards ....................................... 8.8.3 Unbounded Wildcards ........................................................ 8.8.4 Wildcard Capture ................................................................ Reflection and Generics ................................................................... 432 432 433 434 437 438 441 441 442 443 445 447 447 447 448 448 450 451 452 453 454 455 457 459 459 461 464 465 467 From the Library of Hristo Dimov Hristov Contents 8.9.1 8.9.2 8.9.3 8.9.4 The Generic Class Class ....................................................... Using Class<T> Parameters for Type Matching ................... Generic Type Information in the Virtual Machine .......... Type Literals ......................................................................... xiii 467 469 469 473 Chapter 9: Collections ................................................................................ 481 9.1 9.2 9.3 9.4 9.5 9.6 The Java Collections Framework ..................................................... 9.1.1 Separating Collection Interfaces and Implementation .... 9.1.2 The Collection Interface ......................................................... 9.1.3 Iterators ................................................................................. 9.1.4 Generic Utility Methods ...................................................... Interfaces in the Collections Framework ........................................ Concrete Collections ........................................................................ 9.3.1 Linked Lists .......................................................................... 9.3.2 Array Lists ............................................................................ 9.3.3 Hash Sets .............................................................................. 9.3.4 Tree Sets ............................................................................... 9.3.5 Queues and Deques ............................................................ 9.3.6 Priority Queues .................................................................... Maps ................................................................................................... 9.4.1 Basic Map Operations ......................................................... 9.4.2 Updating Map Entries ......................................................... 9.4.3 Map Views ............................................................................ 9.4.4 Weak Hash Maps ................................................................. 9.4.5 Linked Hash Sets and Maps ............................................... 9.4.6 Enumeration Sets and Maps ............................................... 9.4.7 Identity Hash Maps ............................................................. Views and Wrappers ......................................................................... 9.5.1 Small Collections ................................................................. 9.5.2 Subranges ............................................................................. 9.5.3 Unmodifiable Views ............................................................ 9.5.4 Synchronized Views ............................................................ 9.5.5 Checked Views .................................................................... 9.5.6 A Note on Optional Operations ........................................ Algorithms ......................................................................................... 9.6.1 Why Generic Algorithms? .................................................. 482 482 485 485 489 492 494 496 507 507 511 516 518 519 519 523 525 526 527 529 530 532 532 534 535 536 536 537 541 541 From the Library of Hristo Dimov Hristov xiv Contents 9.7 9.6.2 9.6.3 9.6.4 9.6.5 9.6.6 9.6.7 Legacy 9.7.1 9.7.2 9.7.3 9.7.4 9.7.5 Sorting and Shuffling .......................................................... Binary Search ....................................................................... Simple Algorithms ............................................................... Bulk Operations ................................................................... Converting between Collections and Arrays .................... Writing Your Own Algorithms .......................................... Collections ............................................................................ The Hashtable Class ................................................................ Enumerations ....................................................................... Property Maps ...................................................................... Stacks .................................................................................... Bit Sets .................................................................................. 543 546 547 549 550 551 552 553 553 555 558 559 Chapter 10: Graphical User Interface Programming ............................... 565 10.1 A History of Java User Interface Toolkits ...................................... 10.2 Displaying Frames ............................................................................. 10.2.1 Creating a Frame ................................................................. 10.2.2 Frame Properties .................................................................. 10.3 Displaying Information in a Component ....................................... 10.3.1 Working with 2D Shapes .................................................... 10.3.2 Using Color .......................................................................... 10.3.3 Using Fonts .......................................................................... 10.3.4 Displaying Images ............................................................... 10.4 Event Handling .................................................................................. 10.4.1 Basic Event Handling Concepts ......................................... 10.4.2 Example: Handling a Button Click .................................... 10.4.3 Specifying Listeners Concisely ........................................... 10.4.4 Adapter Classes ................................................................... 10.4.5 Actions .................................................................................. 10.4.6 Mouse Events ....................................................................... 10.4.7 The AWT Event Hierarchy ................................................. 10.5 The Preferences API ......................................................................... 565 567 568 570 574 579 587 589 597 598 598 600 604 605 608 614 620 624 Chapter 11: User Interface Components with Swing .............................. 631 11.1 Swing and the Model-View-Controller Design Pattern ................ 632 11.2 Introduction to Layout Management .............................................. 636 From the Library of Hristo Dimov Hristov Contents 11.3 11.4 11.5 11.6 11.2.1 Layout Managers ................................................................. 11.2.2 Border Layout ...................................................................... 11.2.3 Grid Layout .......................................................................... Text Input .......................................................................................... 11.3.1 Text Fields ............................................................................ 11.3.2 Labels and Labeling Components ..................................... 11.3.3 Password Fields ................................................................... 11.3.4 Text Areas ............................................................................ 11.3.5 Scroll Panes .......................................................................... Choice Components ......................................................................... 11.4.1 Checkboxes .......................................................................... 11.4.2 Radio Buttons ....................................................................... 11.4.3 Borders .................................................................................. 11.4.4 Combo Boxes ....................................................................... 11.4.5 Sliders ................................................................................... Menus ................................................................................................. 11.5.1 Menu Building ..................................................................... 11.5.2 Icons in Menu Items ........................................................... 11.5.3 Checkbox and Radio Button Menu Items ......................... 11.5.4 Pop-Up Menus ..................................................................... 11.5.5 Keyboard Mnemonics and Accelerators ........................... 11.5.6 Enabling and Disabling Menu Items ................................. 11.5.7 Toolbars ................................................................................ 11.5.8 Tooltips ................................................................................. Sophisticated Layout Management ................................................. 11.6.1 The Grid Bag Layout ........................................................... 11.6.1.1 The gridx, gridy, gridwidth, and gridheight Parameters ........................................................... 11.6.1.2 Weight Fields ....................................................... 11.6.1.3 The fill and anchor Parameters ........................... 11.6.1.4 Padding ................................................................ 11.6.1.5 Alternative Method to Specify the gridx, gridy, gridwidth, and gridheight Parameters ..................... 11.6.1.6 A Grid Bag Layout Recipe ................................. 11.6.1.7 A Helper Class to Tame the Grid Bag Constraints ........................................................... xv 637 639 642 643 643 645 647 647 648 651 651 654 658 661 665 671 672 675 676 677 679 682 687 689 690 691 693 694 694 694 695 695 696 From the Library of Hristo Dimov Hristov xvi Contents 11.6.2 11.7 Dialog 11.7.1 11.7.2 11.7.3 11.7.4 Custom Layout Managers ................................................... Boxes ...................................................................................... Option Dialogs ..................................................................... Creating Dialogs .................................................................. Data Exchange ..................................................................... File Dialogs ........................................................................... 702 706 707 712 716 723 Chapter 12: Concurrency ........................................................................... 733 12.1 What Are Threads? ........................................................................... 12.2 Thread States ..................................................................................... 12.2.1 New Threads ........................................................................ 12.2.2 Runnable Threads ................................................................ 12.2.3 Blocked and Waiting Threads ............................................ 12.2.4 Terminated Threads ............................................................ 12.3 Thread Properties .............................................................................. 12.3.1 Interrupting Threads ........................................................... 12.3.2 Daemon Threads ................................................................. 12.3.3 Thread Names ...................................................................... 12.3.4 Handlers for Uncaught Exceptions .................................... 12.3.5 Thread Priorities .................................................................. 12.4 Synchronization ................................................................................. 12.4.1 An Example of a Race Condition ...................................... 12.4.2 The Race Condition Explained .......................................... 12.4.3 Lock Objects ........................................................................ 12.4.4 Condition Objects ............................................................... 12.4.5 The synchronized Keyword ...................................................... 12.4.6 Synchronized Blocks ........................................................... 12.4.7 The Monitor Concept .......................................................... 12.4.8 Volatile Fields ....................................................................... 12.4.9 Final Variables ..................................................................... 12.4.10 Atomics ................................................................................. 12.4.11 Deadlocks ............................................................................. 12.4.12 Thread-Local Variables ....................................................... 12.4.13 Why the stop and suspend Methods Are Deprecated .......... 12.5 Thread-Safe Collections ................................................................... 12.5.1 Blocking Queues .................................................................. 734 739 740 740 741 742 743 743 746 747 747 749 750 750 752 755 758 764 768 770 771 772 773 775 778 779 781 781 From the Library of Hristo Dimov Hristov Contents 12.5.2 Efficient Maps, Sets, and Queues ...................................... 12.5.3 Atomic Update of Map Entries .......................................... 12.5.4 Bulk Operations on Concurrent Hash Maps .................... 12.5.5 Concurrent Set Views ......................................................... 12.5.6 Copy on Write Arrays ......................................................... 12.5.7 Parallel Array Algorithms ................................................... 12.5.8 Older Thread-Safe Collections ........................................... 12.6 Tasks and Thread Pools ................................................................... 12.6.1 Callables and Futures .......................................................... 12.6.2 Executors .............................................................................. 12.6.3 Controlling Groups of Tasks .............................................. 12.6.4 The Fork-Join Framework ................................................... 12.7 Asynchronous Computations ........................................................... 12.7.1 Completable Futures ........................................................... 12.7.2 Composing Completable Futures ...................................... 12.7.3 Long-Running Tasks in User Interface Callbacks ............ 12.8 Processes ............................................................................................ 12.8.1 Building a Process ............................................................... 12.8.2 Running a Process ............................................................... 12.8.3 Process Handles ................................................................... xvii 789 790 794 796 797 797 799 800 800 802 806 811 814 815 817 823 831 832 834 835 Appendix ...................................................................................................... 839 Index ............................................................................................................. 843 From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov Preface To the Reader In late 1995, the Java programming language burst onto the Internet scene and gained instant celebrity status. The promise of Java technology was that it would become the universal glue that connects users with information wherever it comes from—web servers, databases, information providers, or any other imaginable source. Indeed, Java is in a unique position to fulfill this promise. It is an extremely solidly engineered language that has gained wide acceptance. Its built-in security and safety features are reassuring both to programmers and to the users of Java programs. Java has built-in support for advanced programming tasks, such as network programming, database connectivity, and concurrency. Since 1995, eleven major revisions of the Java Development Kit have been released. Over the course of the last 20 years, the Application Programming Interface (API) has grown from about 200 to over 4,000 classes. The API now spans such diverse areas as user interface construction, database management, internationalization, security, and XML processing. The book that you are reading right now is the first volume of the eleventh edition of Core Java. Each edition closely followed a release of the Java Development Kit, and each time, we rewrote the book to take advantage of the newest Java features. This edition has been updated to reflect the features of Java Standard Edition (SE) 9, 10, and 11. As with the previous editions of this book, we still target serious programmers who want to put Java to work on real projects. We think of you, our reader, as a programmer with a solid background in a programming language other than Java, and we assume that you don’t like books filled with toy examples (such as toasters, zoo animals, or “nervous text”). You won’t find any of these in our book. Our goal is to enable you to fully understand the Java language and library, not to give you an illusion of understanding. In this book you will find lots of sample code demonstrating almost every language and library feature that we discuss. We keep the sample programs purposefully simple to focus on the major points, but, for the most part, they xix From the Library of Hristo Dimov Hristov xx Preface aren’t fake and they don’t cut corners. They should make good starting points for your own code. We assume you are willing, even eager, to learn about all the advanced features that Java puts at your disposal. For example, we give you a detailed treatment of • • • • • • • • • Object-oriented programming Reflection and proxies Interfaces and inner classes Exception handling Generic programming The collections framework The event listener model Graphical user interface design Concurrency With the explosive growth of the Java class library, a one-volume treatment of all the features of Java that serious programmers need to know is no longer possible. Hence, we decided to break up the book into two volumes. This first volume concentrates on the fundamental concepts of the Java language, along with the basics of user-interface programming. The second volume, Core Java, Volume II—Advanced Features, goes further into the enterprise features and advanced user-interface programming. It includes detailed discussions of • • • • • • • • • • The Stream API File processing and regular expressions Databases XML processing Annotations Internationalization Network programming Advanced GUI components Advanced graphics Native methods When writing a book, errors and inaccuracies are inevitable. We’d very much like to know about them. But, of course, we’d prefer to learn about each of them only once. We have put up a list of frequently asked questions, bug fixes, and workarounds on a web page at http://horstmann.com/corejava. Strategically placed at the end of the errata page (to encourage you to read through From the Library of Hristo Dimov Hristov Preface xxi it first) is a form you can use to report bugs and suggest improvements. Please don’t be disappointed if we don’t answer every query or don’t get back to you immediately. We do read all e-mail and appreciate your input to make future editions of this book clearer and more informative. A Tour of This Book Chapter 1 gives an overview of the capabilities of Java that set it apart from other programming languages. We explain what the designers of the language set out to do and to what extent they succeeded. Then, we give a short history of how Java came into being and how it has evolved. In Chapter 2, we tell you how to download and install the JDK and the program examples for this book. Then we guide you through compiling and running a console application and a graphical application. You will see how to use the plain JDK, a Java IDE, and the JShell tool. Chapter 3 starts the discussion of the Java language. In this chapter, we cover the basics: variables, loops, and simple functions. If you are a C or C++ programmer, this is smooth sailing because the syntax for these language features is essentially the same as in C. If you come from a non-C background such as Visual Basic, you will want to read this chapter carefully. Object-oriented programming (OOP) is now in the mainstream of programming practice, and Java is an object-oriented programming language. Chapter 4 introduces encapsulation, the first of two fundamental building blocks of object orientation, and the Java language mechanism to implement it—that is, classes and methods. In addition to the rules of the Java language, we also give advice on sound OOP design. Finally, we cover the marvelous javadoc tool that formats your code comments as a set of hyperlinked web pages. If you are familiar with C++, you can browse through this chapter quickly. Programmers coming from a non-object-oriented background should expect to spend some time mastering the OOP concepts before going further with Java. Classes and encapsulation are only one part of the OOP story, and Chapter 5 introduces the other—namely, inheritance. Inheritance lets you take an existing class and modify it according to your needs. This is a fundamental technique for programming in Java. The inheritance mechanism in Java is quite similar to that in C++. Once again, C++ programmers can focus on the differences between the languages. From the Library of Hristo Dimov Hristov xxii Preface Chapter 6 shows you how to use Java’s notion of an interface. Interfaces let you go beyond the simple inheritance model of Chapter 5. Mastering interfaces allows you to have full access to the power of Java’s completely object-oriented approach to programming. After we cover interfaces, we move on to lambda expressions, a concise way for expressing a block of code that can be executed at a later point in time. We then cover a useful technical feature of Java called inner classes. Chapter 7 discusses exception handling—Java’s robust mechanism to deal with the fact that bad things can happen to good programs. Exceptions give you an efficient way of separating the normal processing code from the error handling. Of course, even after hardening your program by handling all exceptional conditions, it still might fail to work as expected. In the final part of this chapter, we give you a number of useful debugging tips. Chapter 8 gives an overview of generic programming. Generic programming makes your programs easier to read and safer. We show you how to use strong typing and remove unsightly and unsafe casts, and how to deal with the complexities that arise from the need to stay compatible with older versions of Java. The topic of Chapter 9 is the collections framework of the Java platform. Whenever you want to collect multiple objects and retrieve them later, you should use a collection that is best suited for your circumstances, instead of just tossing the elements into an array. This chapter shows you how to take advantage of the standard collections that are prebuilt for your use. Chapter 10 provides an introduction into GUI programming. We show how you can make windows, how to paint on them, how to draw with geometric shapes, how to format text in multiple fonts, and how to display images. Next, you’ll see how to write code that responds to events, such as mouse clicks or key presses. Chapter 11 discusses the Swing GUI toolkit in great detail. The Swing toolkit allows you to build cross-platform graphical user interfaces. You’ll learn all about the various kinds of buttons, text components, borders, sliders, list boxes, menus, and dialog boxes. However, some of the more advanced components are discussed in Volume II. Chapter 12 finishes the book with a discussion of concurrency, which enables you to program tasks to be done in parallel. This is an important and exciting application of Java technology in an era where most processors have multiple cores that you want to keep busy. From the Library of Hristo Dimov Hristov Preface xxiii A bonus JavaFX chapter contains a rapid introduction into JavaFX, a modern GUI toolkit for desktop applications. If you read the print book, download the chapter from the book companion site at http://horstmann.com/corejava. The Appendix lists the reserved words of the Java language. Conventions As is common in many computer books, we use monospace type to represent computer code. NOTE: Notes are tagged with “note” icons that look like this. TIP: Tips are tagged with “tip” icons that look like this. CAUTION: When there is danger ahead, we warn you with a “caution” icon. C++ NOTE: There are many C++ notes that explain the differences between Java and C++. You can skip over them if you don’t have a background in C++ or if you consider your experience with that language a bad dream of which you’d rather not be reminded. Java comes with a large programming library, or Application Programming Interface (API). When using an API call for the first time, we add a short summary description at the end of the section. These descriptions are a bit more informal but, we hope, also a little more informative than those in the official online API documentation. The names of interfaces are in italics, just like in the official documentation. The number after a class, interface, or method name is the JDK version in which the feature was introduced, as shown in the following example: Application Programming Interface 9 From the Library of Hristo Dimov Hristov xxiv Preface Programs whose source code is on the book’s companion web site are presented as listings, for instance: Listing 1.1 InputTest/InputTest.java Sample Code The web site for this book at http://horstmann.com/corejava contains all sample code from the book. See Chapter 2 for more information on installing the Java Development Kit and the sample code. Register your copy of Core Java, Volume I—Fundamentals, Eleventh Edition, on the InformIT site for convenient access to updates and/or corrections as they become available. To start the registration process, go to informit.com/register and log in or create an account. Enter the product ISBN (9780135166307) and click Submit. Look on the Registered Products tab for an Access Bonus Content link next to this product, and follow that link to access any available bonus materials. If you would like to be notified of exclusive offers on new editions and updates, please check the box to receive email from us. From the Library of Hristo Dimov Hristov Acknowledgments Writing a book is always a monumental effort, and rewriting it doesn’t seem to be much easier, especially with the continuous change in Java technology. Making a book a reality takes many dedicated people, and it is my great pleasure to acknowledge the contributions of the entire Core Java team. A large number of individuals at Pearson provided valuable assistance but managed to stay behind the scenes. I’d like them all to know how much I appreciate their efforts. As always, my warm thanks go to my editor, Greg Doench, for steering the book through the writing and production process, and for allowing me to be blissfully unaware of the existence of all those folks behind the scenes. I am very grateful to Julie Nahil for production support, and to Dmitry Kirsanov and Alina Kirsanova for copyediting and typesetting the manuscript. My thanks also to my coauthor of earlier editions, Gary Cornell, who has since moved on to other ventures. Thanks to the many readers of earlier editions who reported embarrassing errors and made lots of thoughtful suggestions for improvement. I am particularly grateful to the excellent reviewing team who went over the manuscript with an amazing eye for detail and saved me from many embarrassing errors. Reviewers of this and earlier editions include Chuck Allison (Utah Valley University), Lance Andersen (Oracle), Paul Anderson (Anderson Software Group), Alec Beaton (IBM), Cliff Berg, Andrew Binstock (Oracle), Joshua Bloch, David Brown, Corky Cartwright, Frank Cohen (PushToTest), Chris Crane (devXsolution), Dr. Nicholas J. De Lillo (Manhattan College), Rakesh Dhoopar (Oracle), David Geary (Clarity Training), Jim Gish (Oracle), Brian Goetz (Oracle), Angela Gordon, Dan Gordon (Electric Cloud), Rob Gordon, John Gray (University of Hartford), Cameron Gregory (olabs.com), Marty Hall (coreservlets.com, Inc.), Vincent Hardy (Adobe Systems), Dan Harkey (San Jose State University), William Higgins (IBM), Vladimir Ivanovic (PointBase), Jerry Jackson (CA Technologies), Tim Kimmet (Walmart), Chris Laffra, Charlie Lai (Apple), Angelika Langer, Doug Langston, Hang Lau (McGill University), Mark Lawrence, Doug Lea (SUNY Oswego), Gregory Longshore, Bob Lynch (Lynch Associates), Philip Milne (consultant), Mark Morrissey (The Oregon Graduate Institute), Mahesh Neelakanta (Florida Atlantic University), Hao Pham, Paul Philion, Blake Ragsdell, Stuart Reges (University of Arizona), Simon Ritter (Azul Systems), Rich Rosen (Interactive Data Corporation), Peter Sanders (ESSI University, Nice, France), Dr. Paul Sanghera (San xxv From the Library of Hristo Dimov Hristov xxvi Acknowledgments Jose State University and Brooks College), Paul Sevinc (Teamup AG), Devang Shah (Sun Microsystems), Yoshiki Shibata, Bradley A. Smith, Steven Stelting (Oracle), Christopher Taylor, Luke Taylor (Valtech), George Thiruvathukal, Kim Topley (StreamingEdge), Janet Traub, Paul Tyma (consultant), Peter van der Linden, Christian Ullenboom, Burt Walsh, Dan Xu (Oracle), and John Zavgren (Oracle). Cay Horstmann San Francisco, California June 2018 From the Library of Hristo Dimov Hristov CHAPTER 1 An Introduction to Java In this chapter • 1.1 Java as a Programming Platform, page 1 • 1.2 The Java “White Paper” Buzzwords, page 2 • 1.3 Java Applets and the Internet, page 9 • 1.4 A Short History of Java, page 10 • 1.5 Common Misconceptions about Java, page 13 The first release of Java in 1996 generated an incredible amount of excitement, not just in the computer press, but in mainstream media such as the New York Times, the Washington Post, and BusinessWeek. Java has the distinction of being the first and only programming language that had a ten-minute story on National Public Radio. A $100,000,000 venture capital fund was set up solely for products using a specific computer language. I hope you will enjoy a brief history of Java that you will find in this chapter. 1.1 Java as a Programming Platform In the first edition of this book, my coauthor Gary Cornell and I had this to write about Java: “As a computer language, Java’s hype is overdone: Java is certainly a good programming language. There is no doubt that it is one of the better languages available to serious programmers. We think it could potentially have been a 1 From the Library of Hristo Dimov Hristov 2 Chapter 1 An Introduction to Java great programming language, but it is probably too late for that. Once a language is out in the field, the ugly reality of compatibility with existing code sets in.” Our editor got a lot of flack for this paragraph from someone very high up at Sun Microsystems, the company that originally developed Java. The Java language has a lot of nice features that we will examine in detail later in this chapter. It has its share of warts, and some of the newer additions to the language are not as elegant as the original features because of compatibility requirements. But, as we already said in the first edition, Java was never just a language. There are lots of programming languages out there, but few of them make much of a splash. Java is a whole platform, with a huge library, containing lots of reusable code, and an execution environment that provides services such as security, portability across operating systems, and automatic garbage collection. As a programmer, you will want a language with a pleasant syntax and comprehensible semantics (i.e., not C++). Java fits the bill, as do dozens of other fine languages. Some languages give you portability, garbage collection, and the like, but they don’t have much of a library, forcing you to roll your own if you want fancy graphics or networking or database access. Well, Java has everything—a good language, a high-quality execution environment, and a vast library. That combination is what makes Java an irresistible proposition to so many programmers. 1.2 The Java “White Paper” Buzzwords The authors of Java wrote an influential white paper that explains their design goals and accomplishments. They also published a shorter overview that is organized along the following 11 buzzwords: 1. 2. 3. 4. 5. 6. 7. 8. Simple Object-Oriented Distributed Robust Secure Architecture-Neutral Portable Interpreted From the Library of Hristo Dimov Hristov 1.2 The Java “White Paper” Buzzwords 3 9. High-Performance 10. Multithreaded 11. Dynamic In the following subsections, you will find a summary, with excerpts from the white paper, of what the Java designers say about each buzzword, together with a commentary based on my experiences with the current version of Java. NOTE: The white paper can be found at www.oracle.com/technetwork/java /langenv-140151.html. You can retrieve the overview with the 11 buzzwords at http://horstmann.com/corejava/java-an-overview/7Gosling.pdf. 1.2.1 Simple We wanted to build a system that could be programmed easily without a lot of esoteric training and which leveraged today’s standard practice. So even though we found that C++ was unsuitable, we designed Java as closely to C++ as possible in order to make the system more comprehensible. Java omits many rarely used, poorly understood, confusing features of C++ that, in our experience, bring more grief than benefit. The syntax for Java is, indeed, a cleaned-up version of C++ syntax. There is no need for header files, pointer arithmetic (or even a pointer syntax), structures, unions, operator overloading, virtual base classes, and so on. (See the C++ notes interspersed throughout the text for more on the differences between Java and C++.) The designers did not, however, attempt to fix all of the clumsy features of C++. For example, the syntax of the switch statement is unchanged in Java. If you know C++, you will find the transition to the Java syntax easy. At the time Java was released, C++ was actually not the most commonly used programming language. Many developers used Visual Basic and its drag-anddrop programming environment. These developers did not find Java simple. It took several years for Java development environments to catch up. Nowadays, Java development environments are far ahead of those for most other programming languages. Another aspect of being simple is being small. One of the goals of Java is to enable the construction of software that can run stand-alone on small machines. The size of the basic interpreter and class support is about 40K; the basic standard libraries and thread support (essentially a self-contained microkernel) add another 175K. From the Library of Hristo Dimov Hristov 4 Chapter 1 An Introduction to Java This was a great achievement at the time. Of course, the library has since grown to huge proportions. There is now a separate Java Micro Edition with a smaller library, suitable for embedded devices. 1.2.2 Object-Oriented Simply stated, object-oriented design is a programming technique that focuses on the data—objects—and on the interfaces to those objects. To make an analogy with carpentry, an “object-oriented” carpenter would be mostly concerned with the chair he is building, and secondarily with the tools used to make it; a “nonobject-oriented” carpenter would think primarily of his tools. The object-oriented facilities of Java are essentially those of C++. Object orientation was pretty well established when Java was developed. The object-oriented features of Java are comparable to those of C++. The major difference between Java and C++ lies in multiple inheritance, which Java has replaced with a simpler concept of interfaces. Java has a richer capacity for runtime introspection (discussed in Chapter 5) than C++. 1.2.3 Distributed Java has an extensive library of routines for coping with TCP/IP protocols like HTTP and FTP. Java applications can open and access objects across the Net via URLs with the same ease as when accessing a local file system. Nowadays, one takes this for granted—but in 1995, connecting to a web server from a C++ or Visual Basic program was a major undertaking. 1.2.4 Robust Java is intended for writing programs that must be reliable in a variety of ways. Java puts a lot of emphasis on early checking for possible problems, later dynamic (runtime) checking, and eliminating situations that are error-prone. . . . The single biggest difference between Java and C/C++ is that Java has a pointer model that eliminates the possibility of overwriting memory and corrupting data. The Java compiler detects many problems that in other languages would show up only at runtime. As for the second point, anyone who has spent hours chasing memory corruption caused by a pointer bug will be very happy with this aspect of Java. From the Library of Hristo Dimov Hristov 1.2 The Java “White Paper” Buzzwords 5 1.2.5 Secure Java is intended to be used in networked/distributed environments. Toward that end, a lot of emphasis has been placed on security. Java enables the construction of virus-free, tamper-free systems. From the beginning, Java was designed to make certain kinds of attacks impossible, among them: • Overrunning the runtime stack—a common attack of worms and viruses • Corrupting memory outside its own process space • Reading or writing files without permission Originally, the Java attitude towards downloaded code was “Bring it on!” Untrusted code was executed in a sandbox environment where it could not impact the host system. Users were assured that nothing bad could happen because Java code, no matter where it came from, could never escape from the sandbox. However, the security model of Java is complex. Not long after the first version of the Java Development Kit was shipped, a group of security experts at Princeton University found subtle bugs that allowed untrusted code to attack the host system. Initially, security bugs were fixed quickly. Unfortunately, over time, hackers got quite good at spotting subtle flaws in the implementation of the security architecture. Sun, and then Oracle, had a tough time keeping up with bug fixes. After a number of high-profile attacks, browser vendors and Oracle became increasingly cautious. Java browser plug-ins no longer trust remote code unless it is digitally signed and users have agreed to its execution. NOTE: Even though in hindsight, the Java security model was not as successful as originally envisioned, Java was well ahead of its time. A competing code delivery mechanism from Microsoft relied on digital signatures alone for security. Clearly this was not sufficient: As any user of Microsoft’s own products can confirm, programs from well-known vendors do crash and create damage. From the Library of Hristo Dimov Hristov 6 Chapter 1 An Introduction to Java 1.2.6 Architecture-Neutral The compiler generates an architecture-neutral object file format. The compiled code is executable on many processors, given the presence of the Java runtime system. The Java compiler does this by generating bytecode instructions which have nothing to do with a particular computer architecture. Rather, they are designed to be both easy to interpret on any machine and easy to translate into native machine code on the fly. Generating code for a “virtual machine” was not a new idea at the time. Programming languages such as Lisp, Smalltalk, and Pascal had employed this technique for many years. Of course, interpreting virtual machine instructions is slower than running machine instructions at full speed. However, virtual machines have the option of translating the most frequently executed bytecode sequences into machine code—a process called just-in-time compilation. Java’s virtual machine has another advantage. It increases security because it can check the behavior of instruction sequences. 1.2.7 Portable Unlike C and C++, there are no “implementation-dependent” aspects of the specification. The sizes of the primitive data types are specified, as is the behavior of arithmetic on them. For example, an int in Java is always a 32-bit integer. In C/C++, int can mean a 16-bit integer, a 32-bit integer, or any other size that the compiler vendor likes. The only restriction is that the int type must have at least as many bytes as a short int and cannot have more bytes than a long int. Having a fixed size for number types eliminates a major porting headache. Binary data is stored and transmitted in a fixed format, eliminating confusion about byte ordering. Strings are saved in a standard Unicode format. The libraries that are a part of the system define portable interfaces. For example, there is an abstract Window class and implementations of it for UNIX, Windows, and the Macintosh. The example of a Window class was perhaps poorly chosen. As anyone who has ever tried knows, it is an effort of heroic proportions to implement a user interface that looks good on Windows, the Macintosh, and ten flavors of UNIX. Java 1.0 made the heroic effort, delivering a simple toolkit that provided common user interface elements on a number of platforms. Unfortunately, the result was a library that, with a lot of work, could give barely acceptable From the Library of Hristo Dimov Hristov 1.2 The Java “White Paper” Buzzwords 7 results on different systems. That initial user interface toolkit has since been replaced, and replaced again, and portability across platforms remains an issue. However, for everything that isn’t related to user interfaces, the Java libraries do a great job of letting you work in a platform-independent manner. You can work with files, regular expressions, XML, dates and times, databases, network connections, threads, and so on, without worrying about the underlying operating system. Not only are your programs portable, but the Java APIs are often of higher quality than the native ones. 1.2.8 Interpreted The Java interpreter can execute Java bytecodes directly on any machine to which the interpreter has been ported. Since linking is a more incremental and lightweight process, the development process can be much more rapid and exploratory. This was a real stretch. Anyone who has used Lisp, Smalltalk, Visual Basic, Python, R, or Scala knows what a “rapid and exploratory” development process is. You try out something, and you instantly see the result. For the first 20 years of Java’s existence, development environments were not focused on that experience. It wasn’t until Java 9 that the jshell tool supported rapid and exploratory programming. 1.2.9 High-Performance While the performance of interpreted bytecodes is usually more than adequate, there are situations where higher performance is required. The bytecodes can be translated on the fly (at runtime) into machine code for the particular CPU the application is running on. In the early years of Java, many users disagreed with the statement that the performance was “more than adequate.” Today, however, the just-in-time compilers have become so good that they are competitive with traditional compilers and, in some cases, even outperform them because they have more information available. For example, a just-in-time compiler can monitor which code is executed frequently and optimize just that code for speed. A more sophisticated optimization is the elimination (or “inlining”) of function calls. The just-in-time compiler knows which classes have been loaded. It can use inlining when, based upon the currently loaded collection of classes, a particular function is never overridden, and it can undo that optimization later if necessary. From the Library of Hristo Dimov Hristov 8 Chapter 1 An Introduction to Java 1.2.10 Multithreaded [The] benefits of multithreading are better interactive responsiveness and real-time behavior. Nowadays, we care about concurrency because Moore’s law has come to an end. Instead of faster processors, we just get more of them, and we have to keep them busy. Yet when you look at most programming languages, they show a shocking disregard for this problem. Java was well ahead of its time. It was the first mainstream language to support concurrent programming. As you can see from the white paper, its motivation was a little different. At the time, multicore processors were exotic, but web programming had just started, and processors spent a lot of time waiting for a response from the server. Concurrent programming was needed to make sure the user interface didn’t freeze. Concurrent programming is never easy, but Java has done a very good job making it manageable. 1.2.11 Dynamic In a number of ways, Java is a more dynamic language than C or C++. It was designed to adapt to an evolving environment. Libraries can freely add new methods and instance variables without any effect on their clients. In Java, finding out runtime type information is straightforward. This is an important feature in situations where code needs to be added to a running program. A prime example is code that is downloaded from the Internet to run in a browser. In C or C++, this is indeed a major challenge, but the Java designers were well aware of dynamic languages that made it easy to evolve a running program. Their achievement was to bring this feature to a mainstream programming language. NOTE: Shortly after the initial success of Java, Microsoft released a product called J++ with a programming language and virtual machine that were almost identical to Java. This effort failed to gain traction, and Microsoft followed through with another language called C# that also has many similarities to Java but runs on a different virtual machine. This book does not cover J++ or C#. From the Library of Hristo Dimov Hristov 1.3 Java Applets and the Internet 9 1.3 Java Applets and the Internet The idea here is simple: Users will download Java bytecodes from the Internet and run them on their own machines. Java programs that work on web pages are called applets. To use an applet, you only need a Java-enabled web browser, which will execute the bytecodes for you. You need not install any software. You get the latest version of the program whenever you visit the web page containing the applet. Most importantly, thanks to the security of the virtual machine, you never need to worry about attacks from hostile code. Inserting an applet into a web page works much like embedding an image. The applet becomes a part of the page, and the text flows around the space used for the applet. The point is, this image is alive. It reacts to user commands, changes its appearance, and exchanges data between the computer presenting the applet and the computer serving it. Figure 1.1 shows the Jmol applet that displays molecular structures. By using the mouse, you can rotate and zoom each molecule to better understand its structure. At the time that applets were invented, this kind of direct manipulation was not achievable with web pages—there was only rudimentary JavaScript and no HTML canvas. Figure 1.1 The Jmol applet From the Library of Hristo Dimov Hristov 10 Chapter 1 An Introduction to Java When applets first appeared, they created a huge amount of excitement. Many people believe that the lure of applets was responsible for the astonishing popularity of Java. However, the initial excitement soon turned into frustration. Various versions of the Netscape and Internet Explorer browsers ran different versions of Java, some of which were seriously outdated. This sorry situation made it increasingly difficult to develop applets that took advantage of the most current Java version. Instead, Adobe’s Flash technology became popular for achieving dynamic effects in the browser. Later, when Java was dogged by serious security issues, browsers and the Java browser plug-in became increasingly restrictive. Nowadays, it requires skill and dedication to get applets to work in your browser. For example, if you visit the Jmol web site at http://jmol.sourceforge.net/demo/aminoacids/, you will likely encounter a message exhorting you to configure your browser for allowing applets to run. 1.4 A Short History of Java This section gives a short history of Java’s evolution. It is based on various published sources (most importantly an interview with Java’s creators in the July 1995 issue of SunWorld’s online magazine). Java goes back to 1991, when a group of Sun engineers, led by Patrick Naughton and James Gosling (a Sun Fellow and an all-around computer wizard), wanted to design a small computer language that could be used for consumer devices like cable TV switchboxes. Since these devices do not have a lot of power or memory, the language had to be small and generate very tight code. Also, as different manufacturers may choose different central processing units (CPUs), it was important that the language not be tied to any single architecture. The project was code-named “Green.” The requirements for small, tight, and platform-neutral code led the team to design a portable language that generated intermediate code for a virtual machine. The Sun people came from a UNIX background, so they based their language on C++ rather than Lisp, Smalltalk, or Pascal. But, as Gosling says in the interview, “All along, the language was a tool, not the end.” Gosling decided to call his language “Oak” (presumably because he liked the look of an oak tree that was right outside his window at Sun). The people at Sun later realized that Oak was the name of an existing computer language, so they changed the name to Java. This turned out to be an inspired choice. In 1992, the Green project delivered its first product, called “*7.” It was an extremely intelligent remote control. Unfortunately, no one was interested in producing this at Sun, and the Green people had to find other ways to market From the Library of Hristo Dimov Hristov 1.4 A Short History of Java 11 their technology. However, none of the standard consumer electronics companies were interested either. The group then bid on a project to design a cable TV box that could deal with emerging cable services such as video-ondemand. They did not get the contract. (Amusingly, the company that did was led by the same Jim Clark who started Netscape—a company that did much to make Java successful.) The Green project (with a new name of “First Person, Inc.”) spent all of 1993 and half of 1994 looking for people to buy its technology. No one was found. (Patrick Naughton, one of the founders of the group and the person who ended up doing most of the marketing, claims to have accumulated 300,000 air miles in trying to sell the technology.) First Person was dissolved in 1994. While all of this was going on at Sun, the World Wide Web part of the Internet was growing bigger and bigger. The key to the World Wide Web was the browser translating hypertext pages to the screen. In 1994, most people were using Mosaic, a noncommercial web browser that came out of the supercomputing center at the University of Illinois in 1993. (Mosaic was partially written by Marc Andreessen as an undergraduate student on a work-study project, for $6.85 an hour. He moved on to fame and fortune as one of the cofounders and the chief of technology at Netscape.) In the SunWorld interview, Gosling says that in mid-1994, the language developers realized that “We could build a real cool browser. It was one of the few things in the client/server mainstream that needed some of the weird things we’d done: architecture-neutral, real-time, reliable, secure—issues that weren’t terribly important in the workstation world. So we built a browser.” The actual browser was built by Patrick Naughton and Jonathan Payne and evolved into the HotJava browser, which was designed to show off the power of Java. The browser was capable of executing Java code inside web pages. This “proof of technology” was shown at SunWorld ’95 on May 23, 1995, and inspired the Java craze that continues today. Sun released the first version of Java in early 1996. People quickly realized that Java 1.0 was not going to cut it for serious application development. Sure, you could use Java 1.0 to make a nervous text applet that moved text randomly around in a canvas. But you couldn’t even print in Java 1.0. To be blunt, Java 1.0 was not ready for prime time. Its successor, version 1.1, filled in the most obvious gaps, greatly improved the reflection capability, and added a new event model for GUI programming. It was still rather limited, though. The big news of the 1998 JavaOne conference was the upcoming release of Java 1.2, which replaced the early toylike GUI and graphics toolkits with sophisticated scalable versions. Three days (!) after its release in December From the Library of Hristo Dimov Hristov 12 Chapter 1 An Introduction to Java 1998, Sun’s marketing department changed the name to the catchy Java 2 Standard Edition Software Development Kit Version 1.2. Besides the Standard Edition, two other editions were introduced: the Micro Edition for embedded devices such as cell phones, and the Enterprise Edition for server-side processing. This book focuses on the Standard Edition. Versions 1.3 and 1.4 of the Standard Edition were incremental improvements over the initial Java 2 release, with an ever-growing standard library, increased performance, and, of course, quite a few bug fixes. During this time, much of the initial hype about Java applets and client-side applications abated, but Java became the platform of choice for server-side applications. Version 5.0 was the first release since version 1.1 that updated the Java language in significant ways. (This version was originally numbered 1.5, but the version number jumped to 5.0 at the 2004 JavaOne conference.) After many years of research, generic types (roughly comparable to C++ templates) have been added—the challenge was to add this feature without requiring changes in the virtual machine. Several other useful language features were inspired by C#: a “for each” loop, autoboxing, and annotations. Version 6 (without the .0 suffix) was released at the end of 2006. Again, there were no language changes but additional performance improvements and library enhancements. As datacenters increasingly relied on commodity hardware instead of specialized servers, Sun Microsystems fell on hard times and was purchased by Oracle in 2009. Development of Java stalled for a long time. In 2011, Oracle released a new version, with simple enhancements, as Java 7. In 2014, the release of Java 8 followed, with the most significant changes to the Java language in almost two decades. Java 8 embraces a “functional” style of programming that makes it easy to express computations that can be executed concurrently. All programming languages must evolve to stay relevant, and Java has shown a remarkable capacity to do so. The main feature of Java 9 goes all the way back to 2008. At that time, Mark Reinhold, the chief engineer of the Java platform, started an effort to break up the huge, monolithic Java platform. This was to be achieved by introducing modules, self-contained units of code that provide a specific functionality. It took eleven years to design and implement a module system that is a good fit for the Java platform, and it remains to be seen whether it is also a good fit for Java applications and libraries. Java 9, released in 2017, has other appealing features that we cover in this book. From the Library of Hristo Dimov Hristov 1.5 Common Misconceptions about Java 13 Starting in 2018, Java versions are released every six months, to enable faster introduction of features. Certain versions, such as Java 11, are designated as long-term support versions. Table 1.1 shows the evolution of the Java language and library. As you can see, the size of the application programming interface (API) has grown tremendously. Table 1.1 Evolution of the Java Language Version Year New Language Features Number of Classes and Interfaces 1.0 1996 The language itself 211 1.1 1997 Inner classes 477 1.2 1998 The strictfp modifier 1,524 1.3 2000 None 1,840 1.4 2002 Assertions 2,723 5.0 2004 Generic classes, “for each” loop, varargs, autoboxing, metadata, enumerations, static import 3,279 6 2006 None 3,793 7 2011 Switch with strings, diamond operator, binary literals, exception handling enhancements 4,024 8 2014 Lambda expressions, interfaces with default methods, stream and date/time libraries 4,240 9 2017 Modules, miscellaneous language and library enhancements 6,005 1.5 Common Misconceptions about Java This chapter closes with a commented list of some common misconceptions about Java. Java is an extension of HTML. Java is a programming language; HTML is a way to describe the structure of a web page. They have nothing in common except that there are HTML extensions for placing Java applets on a web page. From the Library of Hristo Dimov Hristov 14 Chapter 1 An Introduction to Java I use XML, so I don’t need Java. Java is a programming language; XML is a way to describe data. You can process XML data with any programming language, but the Java API contains excellent support for XML processing. In addition, many important XML tools are implemented in Java. See Volume II for more information. Java is an easy programming language to learn. No programming language as powerful as Java is easy. You always have to distinguish between how easy it is to write toy programs and how hard it is to do serious work. Also, consider that only seven chapters in this book discuss the Java language. The remaining chapters of both volumes show how to put the language to work, using the Java libraries. The Java libraries contain thousands of classes and interfaces and tens of thousands of functions. Luckily, you do not need to know every one of them, but you do need to know surprisingly many to use Java for anything realistic. Java will become a universal programming language for all platforms. This is possible in theory. But in practice, there are domains where other languages are entrenched. Objective C and its successor, Swift, are not going to be replaced on iOS devices. Anything that happens in a browser is controlled by JavaScript. Windows programs are written in C++ or C#. Java has the edge in server-side programming and in cross-platform client applications. Java is just another programming language. Java is a nice programming language; most programmers prefer it to C, C++, or C#. But there have been hundreds of nice programming languages that never gained widespread popularity, whereas languages with obvious flaws, such as C++ and Visual Basic, have been wildly successful. Why? The success of a programming language is determined far more by the utility of the support system surrounding it than by the elegance of its syntax. Are there useful, convenient, and standard libraries for the features that you need to implement? Are there tool vendors that build great programming and debugging environments? Do the language and the toolset integrate with the rest of the computing infrastructure? Java is successful because its libraries let you easily do things such as networking, web applications, and concurrency. The fact that Java reduces pointer errors is a bonus, so programmers seem to be more productive with Java—but these factors are not the source of its success. Java is proprietary, and should therefore be avoided. When Java was first created, Sun gave free licenses to distributors and end users. Although Sun had ultimate control over Java, they involved many From the Library of Hristo Dimov Hristov 1.5 Common Misconceptions about Java 15 other companies in the development of language revisions and the design of new libraries. Source code for the virtual machine and the libraries has always been freely available, but only for inspection, not for modification and redistribution. Java was “closed source, but playing nice.” This situation changed dramatically in 2007, when Sun announced that future versions of Java would be available under the General Public License (GPL), the same open source license that is used by Linux. Oracle has committed to keeping Java open source. There is only one fly in the ointment—patents. Everyone is given a patent grant to use and modify Java, subject to the GPL, but only on desktop and server platforms. If you want to use Java in embedded systems, you need a different license and will likely need to pay royalties. However, these patents will expire within the next decade, and at that point Java will be entirely free. Java is interpreted, so it is too slow for serious applications. In the early days of Java, the language was interpreted. Nowadays, the Java virtual machine uses a just-in-time compiler. The “hot spots” of your code will run just as fast in Java as they would in C++, and in some cases even faster. All Java programs run inside a web page. All Java applets run inside a web browser. That is the definition of an applet—a Java program running inside a browser. But most Java programs are standalone applications that run outside of a web browser. In fact, many Java programs run on web servers and produce the code for web pages. Java programs are a major security risk. In the early days of Java, there were some well-publicized reports of failures in the Java security system. Researchers viewed it as a challenge to find chinks in the Java armor and to defy the strength and sophistication of the applet security model. The technical failures that they found have all been quickly corrected. Later, there were more serious exploits, to which Sun, and later Oracle, responded too slowly. Browser manufacturers reacted, and perhaps overreacted, by deactivating Java by default. To keep this in perspective, consider the far greater number of virus attacks in Windows executable files that cause real grief but surprisingly little criticism of the weaknesses of the attacked platform. Even 20 years after its creation, Java is far safer than any other commonly available execution platform. JavaScript is a simpler version of Java. JavaScript, a scripting language that can be used inside web pages, was invented by Netscape and originally called LiveScript. JavaScript has a syntax From the Library of Hristo Dimov Hristov 16 Chapter 1 An Introduction to Java that is reminiscent of Java, and the languages’ names sound similar, but otherwise they are unrelated. In particularly, Java is strongly typed—the compiler catches many errors that arise from type misuse. In JavaScript, such errors are only found when the program runs, which makes their elimination far more laborious. With Java, I can replace my desktop computer with a cheap “Internet appliance.” When Java was first released, some people bet big that this was going to happen. Companies produced prototypes of Java-powered network computers, but users were not ready to give up a powerful and convenient desktop for a limited machine with no local storage. Nowadays, of course, the world has changed, and for a large majority of end users, the platform that matters is a mobile phone or tablet. The majority of these devices are controlled by the Android platform, which is a derivative of Java. Learning Java programming will help you with Android programming as well. From the Library of Hristo Dimov Hristov CHAPTER 2 The Java Programming Environment In this chapter • 2.1 Installing the Java Development Kit, page 18 • 2.2 Using the Command-Line Tools, page 23 • 2.3 Using an Integrated Development Environment, page 29 • 2.4 JShell, page 32 In this chapter, you will learn how to install the Java Development Kit (JDK) and how to compile and run various types of programs: console programs, graphical applications, and applets. You can run the JDK tools by typing commands in a terminal window. However, many programmers prefer the comfort of an integrated development environment. You will learn how to use a freely available development environment to compile and run Java programs. Once you have mastered the techniques in this chapter and picked your development tools, you are ready to move on to Chapter 3, where you will begin exploring the Java programming language. 17 From the Library of Hristo Dimov Hristov 18 Chapter 2 The Java Programming Environment 2.1 Installing the Java Development Kit The most complete and up-to-date versions of the Java Development Kit (JDK) are available from Oracle for Linux, Mac OS, Solaris, and Windows. Versions in various states of development exist for many other platforms, but those versions are licensed and distributed by the vendors of those platforms. 2.1.1 Downloading the JDK To download the Java Development Kit, visit the web site at www.oracle.com /technetwork/java/javase/downloads and be prepared to decipher an amazing amount of jargon before you can get the software you need. See Table 2.1 for a summary. Table 2.1 Java Jargon Name Acronym Explanation Java Development Kit JDK The software for programmers who want to write Java programs Java Runtime Environment JRE The software for consumers who want to run Java programs Server JRE — The software for running Java programs on servers Standard Edition SE The Java platform for use on desktops and simple server applications Enterprise Edition EE The Java platform for complex server applications Micro Edition ME The Java platform for use on small devices JavaFX — An alternate toolkit for graphical user interfaces that is included with certain Java SE distributions prior to Java 11 OpenJDK — A free and open source implementation of Java SE Java 2 J2 An outdated term that described Java versions from 1998 until 2006 Software Development Kit SDK An outdated term that described the JDK from 1998 until 2006 Update u Oracle’s term for a bug fix release up to Java 8 NetBeans — Oracle’s integrated development environment From the Library of Hristo Dimov Hristov 2.1 Installing the Java Development Kit 19 You already saw the abbreviation JDK for Java Development Kit. Somewhat confusingly, versions 1.2 through 1.4 of the kit were known as the Java SDK (Software Development Kit). You will still find occasional references to the old term. Up to Java 10, there is also a Java Runtime Environment (JRE) that contains only the virtual machine. That is not what you want as a developer. It is intended for end users who have no need for the compiler. Next, you’ll see the term Java SE everywhere. That is the Java Standard Edition, in contrast to Java EE (Enterprise Edition) and Java ME (Micro Edition). You might run into the term Java 2 that was coined in 1998 when the marketing folks at Sun felt that a fractional version number increment did not properly communicate the momentous advances of JDK 1.2. However, since they had that insight only after the release, they decided to keep the version number 1.2 for the development kit. Subsequent releases were numbered 1.3, 1.4, and 5.0. The platform, however, was renamed from Java to Java 2. Thus, we had Java 2 Standard Edition Software Development Kit Version 5.0, or J2SE SDK 5.0. Fortunately, in 2006, the numbering was simplified. The next version of the Java Standard Edition was called Java SE 6, followed by Java SE 7 and Java SE 8. However, the “internal” version numbers are 1.6.0, 1.7.0, and 1.8.0. This minor madness finally ran its course with Java SE 9, when the version number became 9, and then 9.0.1. (Why not 9.0.0 for the initial version? To keep a modicum of excitement, the version number specification requires that trailing zeroes are dropped for the fleeting interval between a major release and its first security update.) NOTE: For the remainder of the book, we will drop the “SE” acronym. When you see “Java 9”, that means “Java SE 9”. Prior to Java 9, there were 32-bit and 64-bit versions of the Java Development Kit. The 32-bit versions are no longer developed by Oracle. You need to have a 64-bit operating system to use the Oracle JDK. With Linux, you have a choice between an RPM file and a .tar.gz file. We recommend the latter—you can simply uncompress it anywhere you like. Now you know how to pick the right JDK. To summarize: From the Library of Hristo Dimov Hristov 20 Chapter 2 The Java Programming Environment • You want the JDK (Java SE Development Kit), not the JRE. • Linux: Pick the .tar.gz version. Accept the license agreement and download the file. NOTE: Depending on the constellation of the planets, Oracle may offer you a bundle that contains both the Java Development Kit and the NetBeans integrated development environment. I suggest that you stay away from all bundles and install only the Java Development Kit at this time. If you later decide to use NetBeans, simply download it from http://netbeans.org. 2.1.2 Setting up the JDK After downloading the JDK, you need to install it and figure out where it was installed—you’ll need that information later. • Under Windows, launch the setup program. You will be asked where to install the JDK. It is best not to accept a default location with spaces in the path name, such as c:\Program Files\Java\jdk-11.0.x. Just take out the Program Files part of the path name. • On the Mac, run the installer. It installs the software into /Library/Java /JavaVirtualMachines/jdk-11.0.x.jdk/Contents/Home. Locate it with the Finder. • On Linux, simply uncompress the .tar.gz file to a location of your choice, such as your home directory or /opt. Or, if you installed from the RPM file, double-check that it is installed in /usr/java/jdk-11.0.x. In this book, the installation directory is denoted as jdk. For example, when referring to the jdk/bin directory, I mean the directory with a name such as /opt/jdk-11.0.4/bin or c:\Java\jdk-11.0.4\bin. When you install the JDK on Windows or Linux, you need to carry out one additional step: Add the jdk/bin directory to the executable path—the list of directories that the operating system traverses to locate executable files. • On Linux, add a line such as the following to the end of your ~/.bashrc or ~/.bash_profile file: export PATH=jdk/bin:$PATH Be sure to use the correct path to the JDK, such as /opt/jdk-11.0.4. • Under Windows 10, type “environment” into the search bar of the Windows Settings, and select “Edit environment variables for your account” (see Figure 2.1). An Environment Variables dialog should appear. (It may hide From the Library of Hristo Dimov Hristov 2.1 Installing the Java Development Kit 21 Figure 2.1 Setting system properties in Windows 10 behind the Windows Settings dialog. If you can’t find it anywhere, try running sysdm.cpl from the Run dialog that you get by holding down the Windows and R key at the same time, and then select the Advanced tab and click the Environment Variables button.) Locate and select a variable named Path in the User Variables list. Click the Edit button, then the New button, and add an entry with the jdk\bin directory (see Figure 2.2). Save your settings. Any new “Command Prompt” windows that you start will have the correct path. Here is how you test whether you did it right: Start a terminal window. Type the line javac --version and press the Enter key. You should get a display such as this one: javac 11.0.1 From the Library of Hristo Dimov Hristov 22 Chapter 2 The Java Programming Environment Figure 2.2 Setting the Path environment variable in Windows 10 If instead you get a message such as “javac: command not found” or “The name specified is not recognized as an internal or external command, operable program or batch file,” then you need to go back and double-check your installation. 2.1.3 Installing Source Files and Documentation The library source files are delivered in the JDK as a compressed file lib/src.zip. Unpack that file to get access to the source code. Simply do the following: 1. Make sure the JDK is installed and the jdk/bin directory is on the executable path. From the Library of Hristo Dimov Hristov 2.2 2. Using the Command-Line Tools 23 Make a directory javasrc in your home directory. If you like, you can do this from a terminal window. mkdir javasrc 3. 4. Inside the jdk/lib directory, locate the file src.zip. Unzip the src.zip file into the javasrc directory. In a terminal window, you can execute the commands cd javasrc jar xvf jdk/lib/src.zip cd .. TIP: The src.zip file contains the source code for all public libraries. To obtain even more source (for the compiler, the virtual machine, the native methods, and the private helper classes), go to http://openjdk.java.net. The documentation is contained in a compressed file that is separate from the JDK. You can download the documentation from www.oracle.com/technetwork /java/javase/downloads. Follow these steps: 1. 2. Download the documentation zip file. It is called jdk-11.0.x_doc-all.zip. Unzip the file and rename the doc directory into something more descriptive, like javadoc. If you like, you can do this from the command line: jar xvf Downloads/jdk-11.0.x_doc-all.zip mv docs jdk-11-docs 3. In your browser, navigate to jdk-11-docs/index.html and add this page to your bookmarks. You should also install the Core Java program examples. You can download them from http://horstmann.com/corejava. The programs are packaged into a zip file corejava.zip. Just unzip them into your home directory. They will be located in a directory corejava. If you like, you can do this from the command line: jar xvf Downloads/corejava.zip 2.2 Using the Command-Line Tools If your programming experience comes from a development environment such as Microsoft Visual Studio, you are accustomed to a system with a built-in text editor, menus to compile and launch a program, and a debugger. The JDK contains nothing even remotely similar. You do everything by typing in commands in a terminal window. This sounds cumbersome, but it is From the Library of Hristo Dimov Hristov 24 Chapter 2 The Java Programming Environment nevertheless an essential skill. When you first install Java, you will want to troubleshoot your installation before you install a development environment. Moreover, by executing the basic steps yourself, you gain a better understanding of what a development environment does behind your back. However, after you have mastered the basic steps of compiling and running Java programs, you will want to use a professional development environment. You will see how to do that in the following section. Let’s get started the hard way: compiling and launching a Java program from the command line. 1. 2. 3. Open a terminal window. Go to the corejava/v1ch02/Welcome directory. (The corejava directory is where you installed the source code for the book examples, as explained in Section 2.1.3, “Installing Source Files and Documentation,” on p. 22.) Enter the following commands: javac Welcome.java java Welcome You should see the output shown in Figure 2.3 in the terminal window. Figure 2.3 Compiling and running Welcome.java From the Library of Hristo Dimov Hristov 2.2 Using the Command-Line Tools 25 Congratulations! You have just compiled and run your first Java program. What happened? The javac program is the Java compiler. It compiles the file Welcome.java into the file Welcome.class. The java program launches the Java virtual machine. It executes the bytecodes that the compiler placed in the class file. The Welcome program is extremely simple. It merely prints a message to the terminal. You may enjoy looking inside the program, shown in Listing 2.1. You will see how it works in the next chapter. Listing 2.1 Welcome/Welcome.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 /** * This program displays a greeting for the reader. * @version 1.30 2014-02-27 * @author Cay Horstmann */ public class Welcome { public static void main(String[] args) { String greeting = "Welcome to Core Java!"; System.out.println(greeting); for (int i = 0; i < greeting.length(); i++) System.out.print("="); System.out.println(); } } In the age of integrated development environments, many programmers are unfamiliar with running programs in a terminal window. Any number of things can go wrong, leading to frustrating results. Pay attention to the following points: • If you type in the program by hand, make sure you correctly enter the uppercase and lowercase letters. In particular, the class name is Welcome and not welcome or WELCOME. • The compiler requires a file name (Welcome.java). When you run the program, you specify a class name (Welcome) without a .java or .class extension. • If you get a message such as “Bad command or file name” or “javac: command not found”, go back and double-check your installation, in particular the executable path setting. • If javac reports that it cannot find the file Welcome.java, you should check whether that file is present in the directory. From the Library of Hristo Dimov Hristov 26 Chapter 2 The Java Programming Environment Under Linux, check that you used the correct capitalization for Welcome.java. Under Windows, use the dir command, not the graphical Explorer tool. Some text editors (in particular Notepad) insist on adding an extension .txt to every file’s name. If you use Notepad to edit Welcome.java, it will actually save it as Welcome.java.txt. Under the default Windows settings, Explorer conspires with Notepad and hides the .txt extension because it belongs to a “known file type.” In that case, you need to rename the file, using the ren command, or save it again, placing quotes around the file name: "Welcome.java". • If you launch your program and get an error message complaining about a java.lang.NoClassDefFoundError, then carefully check the name of the offending class. If you get a complaint about welcome (with a lowercase w), then you should reissue the java Welcome command with an uppercase W. As always, case matters in Java. If you get a complaint about Welcome/java, it means you accidentally typed java Welcome.java. Reissue the command as java Welcome. • If you typed java Welcome and the virtual machine can’t find the Welcome class, check if someone has set the CLASSPATH environment variable on your system. It is not a good idea to set this variable globally, but some poorly written software installers in Windows do just that. Follow the same procedure as for setting the PATH environment variable, but this time, remove the setting. TIP: The excellent tutorial at http://docs.oracle.com/javase/tutorial/getStarted /cupojava goes into much greater detail about the “gotchas” that beginners can run into. NOTE: In JDK 11, the javac command is not required with a single source file. This feature is intended to support shell scripts starting with a “shebang” line #!/path/to/java. The Welcome program was not terribly exciting. Next, try out a graphical application. This program is a simple image file viewer that loads and displays an image. As before, compile and run the program from the command line. 1. 2. Open a terminal window. Change to the directory corejava/v1ch02/ImageViewer. From the Library of Hristo Dimov Hristov 2.2 3. Using the Command-Line Tools 27 Enter the following: javac ImageViewer.java java ImageViewer A new program window pops up with the ImageViewer application. Now, select File → Open and look for an image file to open. (There are a couple of sample files in the same directory.) The image is displayed (see Figure 2.4). To close the program, click on the Close box in the title bar or select File → Exit from the menu. Figure 2.4 Running the ImageViewer application Have a quick look at the source code (Listing 2.2). The program is substantially longer than the first program, but it is not too complex if you consider how much code it would take in C or C++ to write a similar application. You’ll learn how to write graphical user interfaces like this in Chapter 10. Listing 2.2 ImageViewer/ImageViewer.java 1 2 3 import java.awt.*; import java.io.*; import javax.swing.*; 4 5 6 7 /** * A program for viewing images. * @version 1.31 2018-04-10 (Continues) From the Library of Hristo Dimov Hristov 28 Chapter 2 The Java Programming Environment Listing 2.2 (Continued) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 * @author Cay Horstmann */ public class ImageViewer { public static void main(String[] args) { EventQueue.invokeLater(() -> { var frame = new ImageViewerFrame(); frame.setTitle("ImageViewer"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }); } } 22 23 24 25 26 27 28 29 /** * A frame with a label to show an image. */ class ImageViewerFrame extends JFrame { private static final int DEFAULT_WIDTH = 300; private static final int DEFAULT_HEIGHT = 400; 30 31 32 33 public ImageViewerFrame() { setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT); 34 35 36 37 // use a label to display the images var label = new JLabel(); add(label); 38 39 40 41 // set up the file chooser var chooser = new JFileChooser(); chooser.setCurrentDirectory(new File(".")); 42 43 44 45 // set up the menu bar var menuBar = new JMenuBar(); setJMenuBar(menuBar); 46 47 48 var menu = new JMenu("File"); menuBar.add(menu); 49 50 var openItem = new JMenuItem("Open"); From the Library of Hristo Dimov Hristov 2.3 Using an Integrated Development Environment 29 menu.add(openItem); openItem.addActionListener(event -> { // show file chooser dialog int result = chooser.showOpenDialog(null); 51 52 53 54 55 // if file selected, set it as icon of the label if (result == JFileChooser.APPROVE_OPTION) { String name = chooser.getSelectedFile().getPath(); label.setIcon(new ImageIcon(name)); } 56 57 58 59 60 61 }); 62 63 var exitItem = new JMenuItem("Exit"); menu.add(exitItem); exitItem.addActionListener(event -> System.exit(0)); 64 65 66 } 67 68 } 2.3 Using an Integrated Development Environment In the preceding section, you saw how to compile and run a Java program from the command line. That is a useful skill for troubleshooting, but for most day-to-day work, you should use an integrated development environment. These environments are so powerful and convenient that it simply doesn’t make much sense to labor on without them. Excellent choices are the freely available Eclipse, IntelliJ IDEA, and NetBeans. In this chapter, you will learn how to get started with Eclipse. Of course, if you prefer a different development environment, you can certainly use it with this book. Get started by downloading Eclipse from http://eclipse.org/downloads. Versions exist for Linux, Mac OS X, and Windows. Run the installation program and pick the installation set called “Eclipse IDE for Java Developers”. Here are the steps to write a program with Eclipse. 1. 2. 3. After starting Eclipse, select File → New → Project from the menu. Select “Java Project” from the wizard dialog (see Figure 2.5). Click the Next button. Uncheck the “Use default location” checkbox. Click on Browse and navigate to the corejava/v1ch02/Welcome directory (Figure 2.6). From the Library of Hristo Dimov Hristov 30 Chapter 2 The Java Programming Environment Figure 2.5 The New Project dialog in Eclipse Figure 2.6 Configuring a project in Eclipse From the Library of Hristo Dimov Hristov 2.3 4. 5. Using an Integrated Development Environment 31 Click the Finish button. The project is now created. Click on the triangles in the left pane next to the project until you locate the file Welcome.java, and double-click on it. You should now see a pane with the program code (see Figure 2.7). Figure 2.7 Editing a source file with Eclipse 6. With the right mouse button, click on the project name (Welcome) in the left pane. Select Run → Run As → Java Application. The program output is displayed in the console pane. Presumably, this program does not have typos or bugs. (It was only a few lines of code, after all.) Let us suppose, for the sake of argument, that your code occasionally contains a typo (perhaps even a syntax error). Try it out—ruin your file, for example, by changing the capitalization of String as follows: string greeting = "Welcome to Core Java!"; From the Library of Hristo Dimov Hristov 32 Chapter 2 The Java Programming Environment Note the wiggly line under string. In the tabs below the source code, click on Problems and expand the triangles until you see an error message that complains about an unknown string type (see Figure 2.8). Click on the error message. The cursor moves to the matching line in the edit pane, where you can correct your error. This allows you to fix your errors quickly. Figure 2.8 Error messages in Eclipse TIP: Often, an Eclipse error report is accompanied by a lightbulb icon. Click on the lightbulb to get a list of suggested fixes. 2.4 JShell In the preceding section, you saw how to compile and run a Java program. Java 9 introduces another way of working with Java. The JShell program provides a “read-evaluate-print loop,” or REPL. You type a Java expression; JShell evaluates your input, prints the result, and waits for your next input. To start JShell, simply type jshell in a terminal window (see Figure 2.9). From the Library of Hristo Dimov Hristov 2.4 JShell 33 Figure 2.9 Running JShell JShell starts with a greeting, followed by a prompt: | Welcome to JShell -- Version 11.0.1 | For an introduction type: /help intro jshell> Now type an expression, such as "Core Java".length() JShell responds with the result—in this case, the number of characters in the string “Core Java”. $1 ==> 9 Note that you do not type System.out.println. JShell automatically prints the value of every expression that you enter. From the Library of Hristo Dimov Hristov 34 Chapter 2 The Java Programming Environment The $1 in the output indicates that the result is available in further calculations. For example, if you type 5 * $1 - 3 the response is $2 ==> 42 If you need a variable many times, you can give it a more memorable name. However, you have to follow the Java syntax and specify both the type and the name. (We will cover the syntax in Chapter 3.) For example, jshell> int answer = 6 * 7 answer ==> 42 Another useful feature is tab completion. Type Math. followed by the Tab key. You get a list of all methods that you can invoke with the Math class: jshell> Math. E acos( atan2( copySign( exp( floorMod( incrementExact( max( multiplyHigh( nextUp( round( sinh( tanh( ulp( IEEEremainder( addExact( cbrt( cos( expm1( fma( log( min( negateExact( pow( scalb( sqrt( toDegrees( PI asin( ceil( cosh( floor( getExponent( log10( multiplyExact( nextAfter( random() signum( subtractExact( toIntExact( abs( atan( class decrementExact( floorDiv( hypot( log1p( multiplyFull( nextDown( rint( sin( tan( toRadians( Now type l and hit the Tab key again. The method name is completed to log, and you get a shorter list: jshell> Math.log log( log10( log1p( Now you can fill in the rest by hand: jshell> Math.log10(0.001) $3 ==> -3.0 To repeat a command, hit the ↑ key until you see the line that you want to reissue or edit. You can move the cursor in the line with the ← and → keys, From the Library of Hristo Dimov Hristov 2.4 JShell 35 and add or delete characters. Hit Enter when you are done. For example, hit and replace 0.001 with 1000, then hit Enter: jshell> Math.log10(1000) $4 ==> 3.0 JShell makes it easy and fun to learn about the Java language and library without having to launch a heavy-duty development environment and without fussing with public static void main. In this chapter, you learned about the mechanics of compiling and running Java programs. You are now ready to move on to Chapter 3 where you will start learning the Java language. From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov CHAPTER 3 Fundamental Programming Structures in Java In this chapter • 3.1 A Simple Java Program, page 38 • 3.2 Comments, page 41 • 3.3 Data Types, page 42 • 3.4 Variables and Constants, page 48 • 3.5 Operators, page 52 • 3.6 Strings, page 62 • 3.7 Input and Output, page 75 • 3.8 Control Flow, page 86 • 3.9 Big Numbers, page 105 • 3.10 Arrays, page 108 At this point, we are assuming that you successfully installed the JDK and were able to run the sample programs that we showed you in Chapter 2. 37 From the Library of Hristo Dimov Hristov 38 Chapter 3 Fundamental Programming Structures in Java It’s time to start programming. This chapter shows you how the basic programming concepts such as data types, branches, and loops are implemented in Java. 3.1 A Simple Java Program Let’s look more closely at one of the simplest Java programs you can have—one that merely prints a message to console: public class FirstSample { public static void main(String[] args) { System.out.println("We will not use 'Hello, World!'"); } } It is worth spending all the time you need to become comfortable with the framework of this sample; the pieces will recur in all applications. First and foremost, Java is case sensitive. If you made any mistakes in capitalization (such as typing Main instead of main), the program will not run. Now let’s look at this source code line by line. The keyword public is called an access modifier; these modifiers control the level of access other parts of a program have to this code. We’ll have more to say about access modifiers in Chapter 5. The keyword class reminds you that everything in a Java program lives inside a class. Although we will spend a lot more time on classes in the next chapter, for now think of a class as a container for the program logic that defines the behavior of an application. As mentioned in Chapter 1, classes are the building blocks with which all Java applications and applets are built. Everything in a Java program must be inside a class. Following the keyword class is the name of the class. The rules for class names in Java are quite generous. Names must begin with a letter, and after that, they can have any combination of letters and digits. The length is essentially unlimited. You cannot use a Java reserved word (such as public or class) for a class name. (See the appendix for a list of reserved words.) The standard naming convention (which we follow in the name FirstSample) is that class names are nouns that start with an uppercase letter. If a name consists of multiple words, use an initial uppercase letter in each of the words. (This use of uppercase letters in the middle of a word is sometimes called “camel case” or, self-referentially, “CamelCase”. From the Library of Hristo Dimov Hristov 3.1 A Simple Java Program 39 You need to make the file name for the source code the same as the name of the public class, with the extension .java appended. Thus, you must store this code in a file called FirstSample.java. (Again, case is important—don’t use firstsample.java.) If you have named the file correctly and not made any typos in the source code, then when you compile this source code, you end up with a file containing the bytecodes for this class. The Java compiler automatically names the bytecode file FirstSample.class and stores it in the same directory as the source file. Finally, launch the program by issuing the following command: java FirstSample (Remember to leave off the .class extension.) When the program executes, it simply displays the string We will not use 'Hello, World!' on the console. When you use java ClassName to run a compiled program, the Java virtual machine always starts execution with the code in the main method in the class you indicate. (The term “method” is Java-speak for a function.) Thus, you must have a main method in the source of your class for your code to execute. You can, of course, add your own methods to a class and call them from the main method. (We cover writing your own methods in the next chapter.) NOTE: According to the Java Language Specification, the main method must be declared public. (The Java Language Specification is the official document that describes the Java language. You can view or download it from http://docs.oracle.com/javase/specs. However, several versions of the Java launcher were willing to execute Java programs even when the main method was not public. A programmer filed a bug report. To see it, visit http://bugs.java.com/bugdatabase/index.jsp and enter the Bug ID 4252539. In 1999, that bug was marked as “closed, will not be fixed.” A Sun engineer added an explanation that the Java Virtual Machine Specification (at http://docs.oracle.com/javase/specs/jvms/se8/html) does not mandate that main is public and that “fixing it will cause potential troubles.” Fortunately, sanity finally prevailed. The Java launcher in Java 1.4 and beyond enforces that the main method is public. There are a couple of interesting aspects about this story. On the one hand, it is frustrating to have quality assurance engineers, who are often overworked and not always experts in the fine points of Java, make questionable decisions about bug reports. On the other hand, it is remarkable that Sun made the bug From the Library of Hristo Dimov Hristov 40 Chapter 3 Fundamental Programming Structures in Java reports and their resolutions available for anyone to scrutinize, long before Java was open source. At one point, Sun even let programmers vote for their most despised bugs and used the vote counts to decide which of them would get fixed in the next JDK release. Notice the braces { } in the source code. In Java, as in C/C++, braces delineate the parts (usually called blocks) in your program. In Java, the code for any method must be started by an opening brace { and ended by a closing brace }. Brace styles have inspired an inordinate amount of useless controversy. We follow a style that lines up matching braces. As whitespace is irrelevant to the Java compiler, you can use whatever brace style you like. We will have more to say about the use of braces when we talk about the various kinds of loops. For now, don’t worry about the keywords static void—just think of them as part of what you need to get a Java program to compile. By the end of Chapter 4, you will understand this incantation completely. The point to remember for now is that every Java application must have a main method that is declared in the following way: public class ClassName { public static void main(String[] args) { program statements } } C++ NOTE: As a C++ programmer, you know what a class is. Java classes are similar to C++ classes, but there are a few differences that can trap you. For example, in Java all functions are methods of some class. (The standard terminology refers to them as methods, not member functions.) Thus, in Java you must have a shell class for the main method. You may also be familiar with the idea of static member functions in C++. These are member functions defined inside a class that do not operate on objects. The main method in Java is always static. Finally, as in C/C++, the void keyword indicates that this method does not return a value. Unlike C/C++, the main method does not return an “exit code” to the operating system. If the main method exits normally, the Java program has the exit code 0, indicating successful completion. To terminate the program with a different exit code, use the System.exit method. From the Library of Hristo Dimov Hristov 3.2 Comments 41 Next, turn your attention to this fragment: { System.out.println("We will not use 'Hello, World!'"); } Braces mark the beginning and end of the body of the method. This method has only one statement in it. As with most programming languages, you can think of Java statements as sentences of the language. In Java, every statement must end with a semicolon. In particular, carriage returns do not mark the end of a statement, so statements can span multiple lines if need be. The body of the main method contains a statement that outputs a single line of text to the console. Here, we are using the System.out object and calling its println method. Notice the periods used to invoke a method. Java uses the general syntax object.method(parameters) as its equivalent of a function call. In this case, we are calling the println method and passing it a string parameter. The method displays the string parameter on the console. It then terminates the output line, so that each call to println displays its output on a new line. Notice that Java, like C/C++, uses double quotes to delimit strings. (You can find more information about strings later in this chapter.) Methods in Java, like functions in any programming language, can use zero, one, or more parameters (some programmers call them arguments). Even if a method takes no parameters, you must still use empty parentheses. For example, a variant of the println method with no parameters just prints a blank line. You invoke it with the call System.out.println(); NOTE: System.out also has a print method that doesn’t add a newline character to the output. For example, System.out.print("Hello") prints Hello without a newline. The next output appears immediately after the letter o. 3.2 Comments Comments in Java, as in most programming languages, do not show up in the executable program. Thus, you can add as many comments as needed without fear of bloating the code. Java has three ways of marking comments. From the Library of Hristo Dimov Hristov 42 Chapter 3 Fundamental Programming Structures in Java The most common form is a //. Use this for a comment that runs from the // to the end of the line. System.out.println("We will not use 'Hello, World!'"); // is this too cute? When longer comments are needed, you can mark each line with a //, or you can use the /* and */ comment delimiters that let you block off a longer comment. Finally, a third kind of comment is used to generate documentation automatically. This comment uses a /** to start and a */ to end. You can see this type of comment in Listing 3.1. For more on this type of comment and on automatic documentation generation, see Chapter 4. Listing 3.1 FirstSample/FirstSample.java 1 2 3 4 5 6 7 8 9 10 11 12 /** * This is the first sample program in Core Java Chapter 3 * @version 1.01 1997-03-22 * @author Gary Cornell */ public class FirstSample { public static void main(String[] args) { System.out.println("We will not use 'Hello, World!'"); } } CAUTION: /* */ comments do not nest in Java. That is, you might not be able to deactivate code simply by surrounding it with /* and */ because the code you want to deactivate might itself contain a */ delimiter. 3.3 Data Types Java is a strongly typed language. This means that every variable must have a declared type. There are eight primitive types in Java. Four of them are integer types; two are floating-point number types; one is the character type char, used for code units in the Unicode encoding scheme (see Section 3.3.3, “The char Type,” on p. 46); and one is a boolean type for truth values. From the Library of Hristo Dimov Hristov 3.3 Data Types 43 NOTE: Java has an arbitrary-precision arithmetic package. However, “big numbers,” as they are called, are Java objects and not a primitive Java type. You will see how to use them later in this chapter. 3.3.1 Integer Types The integer types are for numbers without fractional parts. Negative values are allowed. Java provides the four integer types shown in Table 3.1. Table 3.1 Java Integer Types Type Storage Requirement Range (Inclusive) int 4 bytes –2,147,483,648 to 2,147,483,647 (just over 2 billion) short 2 bytes –32,768 to 32,767 long 8 bytes –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 byte 1 byte –128 to 127 In most situations, the int type is the most practical. If you want to represent the number of inhabitants of our planet, you’ll need to resort to a long. The byte and short types are mainly intended for specialized applications, such as low-level file handling, or for large arrays when storage space is at a premium. Under Java, the ranges of the integer types do not depend on the machine on which you will be running the Java code. This alleviates a major pain for the programmer who wants to move software from one platform to another, or even between operating systems on the same platform. In contrast, C and C++ programs use the most efficient integer type for each processor. As a result, a C program that runs well on a 32-bit processor may exhibit integer overflow on a 16-bit system. Since Java programs must run with the same results on all machines, the ranges for the various types are fixed. Long integer numbers have a suffix L or l (for example, 4000000000L). Hexadecimal numbers have a prefix 0x or 0X (for example, 0xCAFE). Octal numbers have a prefix 0 (for example, 010 is 8)—naturally, this can be confusing, so we recommend against the use of octal constants. Starting with Java 7, you can write numbers in binary, with a prefix 0b or 0B. For example, 0b1001 is 9. Also starting with Java 7, you can add underscores to number literals, such as 1_000_000 (or 0b1111_0100_0010_0100_0000) to denote one million. The underscores are for human eyes only. The Java compiler simply removes them. From the Library of Hristo Dimov Hristov 44 Chapter 3 Fundamental Programming Structures in Java C++ NOTE: In C and C++, the sizes of types such as int and long depend on the target platform. On a 16-bit processor such as the 8086, integers are 2 bytes, but on a 32-bit processor like a Pentium or SPARC they are 4-byte quantities. Similarly, long values are 4-byte on 32-bit processors and 8-byte on 64-bit processors. These differences make it challenging to write cross-platform programs. In Java, the sizes of all numeric types are platform-independent. Note that Java does not have any unsigned versions of the int, long, short, or byte types. NOTE: If you work with integer values that can never be negative and you really need an additional bit, you can, with some care, interpret signed integer values as unsigned. For example, instead of having a byte value b represent the range from –128 to 127, you may want a range from 0 to 255. You can store it in a byte. Due to the nature of binary arithmetic, addition, subtraction, and multiplication will work provided they don’t overflow. For other operations, call Byte.toUnsignedInt(b) to get an int value between 0 and 255, then process the integer value and cast back to byte. The Integer and Long classes have methods for unsigned division and remainder. 3.3.2 Floating-Point Types The floating-point types denote numbers with fractional parts. The two floating-point types are shown in Table 3.2. Table 3.2 Floating-Point Types Type Storage Requirement Range float 4 bytes Approximately ±3.40282347E+38F (6–7 significant decimal digits) double 8 bytes Approximately ±1.79769313486231570E+308 (15 significant decimal digits) The name double refers to the fact that these numbers have twice the precision of the float type. (Some people call these double-precision numbers.) The limited precision of float (6–7 significant digits) is simply not sufficient for many situations. Use float values only when you work with a library that requires them, or when you need to store a very large number of them. From the Library of Hristo Dimov Hristov 3.3 Data Types 45 Numbers of type float have a suffix F or f (for example, 3.14F). Floating-point numbers without an F suffix (such as 3.14) are always considered to be of type double. You can optionally supply the D or d suffix (for example, 3.14D). NOTE: You can specify floating-point literals in hexadecimal. For example, 0.125 = 2–3 can be written as 0x1.0p-3. In hexadecimal notation, you use a p, not an e, to denote the exponent. (An e is a hexadecimal digit.) Note that the mantissa is written in hexadecimal and the exponent in decimal. The base of the exponent is 2, not 10. All floating-point computations follow the IEEE 754 specification. In particular, there are three special floating-point values to denote overflows and errors: • Positive infinity • Negative infinity • NaN (not a number) For example, the result of dividing a positive number by 0 is positive infinity. Computing 0/0 or the square root of a negative number yields NaN. NOTE: The constants Double.POSITIVE_INFINITY, Double.NEGATIVE_INFINITY, and Double.NaN (as well as corresponding Float constants) represent these special values, but they are rarely used in practice. In particular, you cannot test if (x == Double.NaN) // is never true to check whether a particular result equals Double.NaN. All “not a number” values are considered distinct. However, you can use the Double.isNaN method: if (Double.isNaN(x)) // check whether x is "not a number" CAUTION: Floating-point numbers are not suitable for financial calculations in which roundoff errors cannot be tolerated. For example, the command System.out.println(2.0 - 1.1) prints 0.8999999999999999, not 0.9 as you would expect. Such roundoff errors are caused by the fact that floating-point numbers are represented in the binary number system. There is no precise binary representation of the fraction 1/10, just as there is no accurate representation of the fraction 1/3 in the decimal system. If you need precise numerical computations without roundoff errors, use the BigDecimal class, which is introduced later in this chapter. From the Library of Hristo Dimov Hristov 46 Chapter 3 Fundamental Programming Structures in Java 3.3.3 The char Type The char type was originally intended to describe individual characters. However, this is no longer the case. Nowadays, some Unicode characters can be described with one char value, and other Unicode characters require two char values. Read the next section for the gory details. Literal values of type char are enclosed in single quotes. For example, 'A' is a character constant with value 65. It is different from "A", a string containing a single character. Values of type char can be expressed as hexadecimal values that run from \u0000 to \uFFFF. For example, \u2122 is the trademark symbol (™) and \u03C0 is the Greek letter pi (π). Besides the \u escape sequences, there are several escape sequences for special characters, as shown in Table 3.3. You can use these escape sequences inside quoted character literals and strings, such as '\u2122' or "Hello\n". The \u escape sequence (but none of the other escape sequences) can even be used outside quoted character constants and strings. For example, public static void main(String\u005B\u005D args) is perfectly legal—\u005B and \u005D are the encodings for [ and ]. Table 3.3 Escape Sequences for Special Characters Escape Sequence Name Unicode Value \b Backspace \u0008 \t Tab \u0009 \n Linefeed \u000a \r Carriage return \u000d \" Double quote \u0022 \' Single quote \u0027 \\ Backslash \u005c CAUTION: Unicode escape sequences are processed before the code is parsed. For example, "\u0022+\u0022" is not a string consisting of a plus sign surrounded by quotation marks (U+0022). Instead, the \u0022 are converted into " before parsing, yielding ""+"", or an empty string. Even more insidiously, you must beware of \u inside comments. The comment // \u000A is a newline From the Library of Hristo Dimov Hristov 3.3 Data Types 47 yields a syntax error since \u000A is replaced with a newline when the program is read. Similarly, a comment // look inside c:\users yields a syntax error because the \u is not followed by four hex digits. 3.3.4 Unicode and the char Type To fully understand the char type, you have to know about the Unicode encoding scheme. Unicode was invented to overcome the limitations of traditional character encoding schemes. Before Unicode, there were many different standards: ASCII in the United States, ISO 8859-1 for Western European languages, KOI-8 for Russian, GB18030 and BIG-5 for Chinese, and so on. This caused two problems. First, a particular code value corresponds to different letters in the different encoding schemes. Second, the encodings for languages with large character sets have variable length: Some common characters are encoded as single bytes, others require two or more bytes. Unicode was designed to solve these problems. When the unification effort started in the 1980s, a fixed 2-byte code was more than sufficient to encode all characters used in all languages in the world, with room to spare for future expansion—or so everyone thought at the time. In 1991, Unicode 1.0 was released, using slightly less than half of the available 65,536 code values. Java was designed from the ground up to use 16-bit Unicode characters, which was a major advance over other programming languages that used 8-bit characters. Unfortunately, over time, the inevitable happened. Unicode grew beyond 65,536 characters, primarily due to the addition of a very large set of ideographs used for Chinese, Japanese, and Korean. Now, the 16-bit char type is insufficient to describe all Unicode characters. We need a bit of terminology to explain how this problem is resolved in Java, beginning with Java 5. A code point is a code value that is associated with a character in an encoding scheme. In the Unicode standard, code points are written in hexadecimal and prefixed with U+, such as U+0041 for the code point of the Latin letter A. Unicode has code points that are grouped into 17 code planes. The first code plane, called the basic multilingual plane, consists of the “classic” Unicode characters with code points U+0000 to U+FFFF. Sixteen additional planes, with code points U+10000 to U+10FFFF, hold the supplementary characters. The UTF-16 encoding represents all Unicode code points in a variable-length code. The characters in the basic multilingual plane are represented as 16-bit values, called code units. The supplementary characters are encoded as consecutive pairs of code units. Each of the values in such an encoding pair falls From the Library of Hristo Dimov Hristov 48 Chapter 3 Fundamental Programming Structures in Java into a range of 2048 unused values of the basic multilingual plane, called the surrogates area (U+D800 to U+DBFF for the first code unit, U+DC00 to U+DFFF for the second code unit). This is rather clever, because you can immediately tell whether a code unit encodes a single character or it is the first or second part of a supplementary character. For example, (the mathematical symbol for the set of octonions, http://math.ucr.edu/home/baez/octonions) has code point U+1D546 and is encoded by the two code units U+D835 and U+DD46. (See https://tools.ietf.org /html/rfc2781 for a description of the encoding algorithm.) In Java, the char type describes a code unit in the UTF-16 encoding. Our strong recommendation is not to use the char type in your programs unless you are actually manipulating UTF-16 code units. You are almost always better off treating strings (which we will discuss in Section 3.6, “Strings,” on p. 62) as abstract data types. 3.3.5 The boolean Type The boolean type has two values, false and true. It is used for evaluating logical conditions. You cannot convert between integers and boolean values. C++ NOTE: In C++, numbers and even pointers can be used in place of boolean values. The value 0 is equivalent to the bool value false, and a nonzero value is equivalent to true. This is not the case in Java. Thus, Java programmers are shielded from accidents such as if (x = 0) // oops... meant x == 0 In C++, this test compiles and runs, always evaluating to false. In Java, the test does not compile because the integer expression x = 0 cannot be converted to a boolean value. 3.4 Variables and Constants As in every programming language, variables are used to store values. Constants are variables whose values don’t change. In the following sections, you will learn how to declare variables and constants. 3.4.1 Declaring Variables In Java, every variable has a type. You declare a variable by placing the type first, followed by the name of the variable. Here are some examples: From the Library of Hristo Dimov Hristov 3.4 Variables and Constants 49 double salary; int vacationDays; long earthPopulation; boolean done; Notice the semicolon at the end of each declaration. The semicolon is necessary because a declaration is a complete Java statement, and all Java statements end in semicolons. A variable name must begin with a letter and must be a sequence of letters or digits. Note that the terms “letter” and “digit” are much broader in Java than in most languages. A letter is defined as 'A'–'Z', 'a'–'z', '_', '$', or any Unicode character that denotes a letter in a language. For example, German users can use umlauts such as 'ä' in variable names; Greek speakers could use a π. Similarly, digits are '0'–'9' and any Unicode characters that denote a digit in a language. Symbols like '+' or '©' cannot be used inside variable names, nor can spaces. All characters in the name of a variable are significant and case is also significant. The length of a variable name is essentially unlimited. TIP: If you are really curious as to what Unicode characters are “letters” as far as Java is concerned, you can use the isJavaIdentifierStart and isJavaIdentifierPart methods in the Character class to check. TIP: Even though $ is a valid Java letter, you should not use it in your own code. It is intended for names that are generated by the Java compiler and other tools. You also cannot use a Java reserved word as a variable name. As of Java 9, a single underscore _ cannot be used as a variable name. A future version of Java may use _ as a wildcard symbol. You can declare multiple variables on a single line: int i, j; // both are integers However, we don’t recommend this style. If you declare each variable separately, your programs are easier to read. NOTE: As you saw, names are case sensitive, for example, hireday and hireDay are two separate names. In general, you should not have two names that only differ in their letter case. However, sometimes it is difficult to come up with a From the Library of Hristo Dimov Hristov 50 Chapter 3 Fundamental Programming Structures in Java good name for a variable. Many programmers then give the variable the same name as the type, for example Box box; // "Box" is the type and "box" is the variable name Other programmers prefer to use an “a” prefix for the variable: Box aBox; 3.4.2 Initializing Variables After you declare a variable, you must explicitly initialize it by means of an assignment statement—you can never use the value of an uninitialized variable. For example, the Java compiler flags the following sequence of statements as an error: int vacationDays; System.out.println(vacationDays); // ERROR--variable not initialized You assign to a previously declared variable by using the variable name on the left, an equal sign (=), and then some Java expression with an appropriate value on the right. int vacationDays; vacationDays = 12; You can both declare and initialize a variable on the same line. For example: int vacationDays = 12; Finally, in Java you can put declarations anywhere in your code. For example, the following is valid code in Java: double salary = 65000.0; System.out.println(salary); int vacationDays = 12; // OK to declare a variable here In Java, it is considered good style to declare variables as closely as possible to the point where they are first used. NOTE: Starting with Java 10, you do not need to declare the types of local variables if they can be inferred from the initial value. Simply use the keyword var instead of the type: var vacationDays = 12; // vacationDays is an int var greeting = "Hello"; // greeting is a String We will start using this feature in the next chapter. From the Library of Hristo Dimov Hristov 3.4 Variables and Constants 51 C++ NOTE: C and C++ distinguish between the declaration and definition of a variable. For example, int i = 10; is a definition, whereas extern int i; is a declaration. In Java, no declarations are separate from definitions. 3.4.3 Constants In Java, you use the keyword final to denote a constant. For example: public class Constants { public static void main(String[] args) { final double CM_PER_INCH = 2.54; double paperWidth = 8.5; double paperHeight = 11; System.out.println("Paper size in centimeters: " + paperWidth * CM_PER_INCH + " by " + paperHeight * CM_PER_INCH); } } The keyword final indicates that you can assign to the variable once, and then its value is set once and for all. It is customary to name constants in all uppercase. It is probably more common in Java to create a constant so it’s available to multiple methods inside a single class. These are usually called class constants. Set up a class constant with the keywords static final. Here is an example of using a class constant: public class Constants2 { public static final double CM_PER_INCH = 2.54; public static void main(String[] args) { double paperWidth = 8.5; double paperHeight = 11; System.out.println("Paper size in centimeters: " + paperWidth * CM_PER_INCH + " by " + paperHeight * CM_PER_INCH); } } From the Library of Hristo Dimov Hristov 52 Chapter 3 Fundamental Programming Structures in Java Note that the definition of the class constant appears outside the main method. Thus, the constant can also be used in other methods of the same class. Furthermore, if the constant is declared, as in our example, public, methods of other classes can also use it—in our example, as Constants2.CM_PER_INCH. C++ NOTE: const is a reserved Java keyword, but it is not currently used for anything. You must use final for a constant. 3.4.4 Enumerated Types Sometimes, a variable should only hold a restricted set of values. For example, you may sell clothes or pizza in four sizes: small, medium, large, and extra large. Of course, you could encode these sizes as integers 1, 2, 3, 4 or characters S, M, L, and X. But that is an error-prone setup. It is too easy for a variable to hold a wrong value (such as 0 or m). You can define your own enumerated type whenever such a situation arises. An enumerated type has a finite number of named values. For example, enum Size { SMALL, MEDIUM, LARGE, EXTRA_LARGE }; Now you can declare variables of this type: Size s = Size.MEDIUM; A variable of type Size can hold only one of the values listed in the type declaration, or the special value null that indicates that the variable is not set to any value at all. We discuss enumerated types in greater detail in Chapter 5. 3.5 Operators Operators are used to combine values. As you will see in the following sections, Java has a rich set of arithmetic and logical operators and mathematical functions. 3.5.1 Arithmetic Operators The usual arithmetic operators +, -, *, / are used in Java for addition, subtraction, multiplication, and division. The / operator denotes integer division if both arguments are integers, and floating-point division otherwise. Integer remainder (sometimes called modulus) is denoted by %. For example, 15 / 2 is 7, 15 % 2 is 1, and 15.0 / 2 is 7.5. From the Library of Hristo Dimov Hristov 3.5 Operators 53 Note that integer division by 0 raises an exception, whereas floating-point division by 0 yields an infinite or NaN result. NOTE: One of the stated goals of the Java programming language is portability. A computation should yield the same results no matter which virtual machine executes it. For arithmetic computations with floating-point numbers, it is surprisingly difficult to achieve this portability. The double type uses 64 bits to store a numeric value, but some processors use 80-bit floating-point registers. These registers yield added precision in intermediate steps of a computation. For example, consider the following computation: double w = x * y / z; Many Intel processors compute x * y, leave the result in an 80-bit register, then divide by z, and finally truncate the result back to 64 bits. That can yield a more accurate result, and it can avoid exponent overflow. But the result may be different from a computation that uses 64 bits throughout. For that reason, the initial specification of the Java virtual machine mandated that all intermediate computations must be truncated. The numeric community hated it. Not only can the truncated computations cause overflow, they are actually slower than the more precise computations because the truncation operations take time. For that reason, the Java programming language was updated to recognize the conflicting demands for optimum performance and perfect reproducibility. By default, virtual machine designers are now permitted to use extended precision for intermediate computations. However, methods tagged with the strictfp keyword must use strict floating-point operations that yield reproducible results. For example, you can tag main as public static strictfp void main(String[] args) Then all instructions inside the main method will use strict floating-point computations. If you tag a class as strictfp, then all of its methods must use strict floating-point computations. The gory details are very much tied to the behavior of the Intel processors. In the default mode, intermediate results are allowed to use an extended exponent, but not an extended mantissa. (The Intel chips support truncation of the mantissa without loss of performance.) Therefore, the only difference between the default and strict modes is that strict computations may overflow when default computations don’t. If your eyes glazed over when reading this note, don’t worry. Floating-point overflow isn’t a problem that one encounters for most common programs. We don’t use the strictfp keyword in this book. From the Library of Hristo Dimov Hristov 54 Chapter 3 Fundamental Programming Structures in Java 3.5.2 Mathematical Functions and Constants The Math class contains an assortment of mathematical functions that you may occasionally need, depending on the kind of programming that you do. To take the square root of a number, use the sqrt method: double x = 4; double y = Math.sqrt(x); System.out.println(y); // prints 2.0 NOTE: There is a subtle difference between the println method and the sqrt method. The println method operates on the System.out object. But the sqrt method in the Math class does not operate on any object. Such a method is called a static method. You can learn more about static methods in Chapter 4. The Java programming language has no operator for raising a quantity to a power: You must use the pow method in the Math class. The statement double y = Math.pow(x, a); sets y to be x raised to the power a (xa). The pow method’s parameters are both of type double, and it returns a double as well. The floorMod method aims to solve a long-standing problem with integer remainders. Consider the expression n % 2. Everyone knows that this is 0 if n is even and 1 if n is odd. Except, of course, when n is odd and negative. Then it is -1. Why? When the first computers were built, someone had to make rules for how integer division and remainder should work for negative operands. Mathematicians had known the optimal (or “Euclidean”) rule for a few hundred years: always leave the remainder ≥ 0. But, rather than open a math textbook, those pioneers came up with rules that seemed reasonable but are actually inconvenient. Consider this problem. You compute the position of the hour hand of a clock. An adjustment is applied, and you want to normalize to a number between 0 and 11. That is easy: (position + adjustment) % 12. But what if the adjustment is negative? Then you might get a negative number. So you have to introduce a branch, or use ((position + adjustment) % 12 + 12) % 12. Either way, it is a hassle. The floorMod method makes it easier: floorMod(position + adjustment, 12) always yields a value between 0 and 11. (Unfortunately, floorMod gives negative results for negative divisors, but that situation doesn’t often occur in practice.) The Math class supplies the usual trigonometric functions: From the Library of Hristo Dimov Hristov 3.5 Operators 55 Math.sin Math.cos Math.tan Math.atan Math.atan2 and the exponential function with its inverse, the natural logarithm, as well as the decimal logarithm: Math.exp Math.log Math.log10 Finally, two constants denote the closest possible approximations to the mathematical constants π and e: Math.PI Math.E TIP: You can avoid the Math prefix for the mathematical methods and constants by adding the following line to the top of your source file: import static java.lang.Math.*; For example: System.out.println("The square root of \u03C0 is " + sqrt(PI)); We discuss static imports in Chapter 4. NOTE: The methods in the Math class use the routines in the computer’s floatingpoint unit for fastest performance. If completely predictable results are more important than performance, use the StrictMath class instead. It implements the algorithms from the “Freely Distributable Math Library” (www.netlib.org/fdlibm), guaranteeing identical results on all platforms. NOTE: The Math class provides several methods to make integer arithmetic safer. The mathematical operators quietly return wrong results when a computation overflows. For example, one billion times three (1000000000 * 3) evaluates to -1294967296 because the largest int value is just over two billion. If you call Math.multiplyExact(1000000000, 3) instead, an exception is generated. You can catch that exception or let the program terminate rather than quietly continue with a wrong result. There are also methods addExact, subtractExact, incrementExact, decrementExact, negateExact, all with int and long parameters. From the Library of Hristo Dimov Hristov 56 Chapter 3 Fundamental Programming Structures in Java 3.5.3 Conversions between Numeric Types It is often necessary to convert from one numeric type to another. Figure 3.1 shows the legal conversions. Figure 3.1 Legal conversions between numeric types The six solid arrows in Figure 3.1 denote conversions without information loss. The three dotted arrows denote conversions that may lose precision. For example, a large integer such as 123456789 has more digits than the float type can represent. When the integer is converted to a float, the resulting value has the correct magnitude but loses some precision. int n = 123456789; float f = n; // f is 1.23456792E8 When two values are combined with a binary operator (such as n + f where n is an integer and f is a floating-point value), both operands are converted to a common type before the operation is carried out. • If either of the operands is of type double, the other one will be converted to a double. • Otherwise, if either of the operands is of type float, the other one will be converted to a float. From the Library of Hristo Dimov Hristov 3.5 Operators 57 • Otherwise, if either of the operands is of type long, the other one will be converted to a long. • Otherwise, both operands will be converted to an int. 3.5.4 Casts In the preceding section, you saw that int values are automatically converted to double values when necessary. On the other hand, there are obviously times when you want to consider a double as an integer. Numeric conversions are possible in Java, but of course information may be lost. Conversions in which loss of information is possible are done by means of casts. The syntax for casting is to give the target type in parentheses, followed by the variable name. For example: double x = 9.997; int nx = (int) x; Now, the variable nx has the value 9 because casting a floating-point value to an integer discards the fractional part. If you want to round a floating-point number to the nearest integer (which in most cases is a more useful operation), use the Math.round method: double x = 9.997; int nx = (int) Math.round(x); Now the variable nx has the value 10. You still need to use the cast (int) when you call round. The reason is that the return value of the round method is a long, and a long can only be assigned to an int with an explicit cast because there is the possibility of information loss. CAUTION: If you try to cast a number of one type to another that is out of range for the target type, the result will be a truncated number that has a different value. For example, (byte) 300 is actually 44. C++ NOTE: You cannot cast between boolean values and any numeric type. This convention prevents common errors. In the rare case when you want to convert a boolean value to a number, you can use a conditional expression such as b ? 1 : 0. From the Library of Hristo Dimov Hristov 58 Chapter 3 Fundamental Programming Structures in Java 3.5.5 Combining Assignment with Operators There is a convenient shortcut for using binary operators in an assignment. For example, x += 4; is equivalent to x = x + 4; (In general, place the operator to the left of the = sign, such as *= or %=.) NOTE: If the operator yields a value whose type is different from that of the lefthand side, then it is coerced to fit. For example, if x is an int, then the statement x += 3.5; is valid, setting x to (int)(x + 3.5). 3.5.6 Increment and Decrement Operators Programmers, of course, know that one of the most common operations with a numeric variable is to add or subtract 1. Java, following in the footsteps of C and C++, has both increment and decrement operators: n++ adds 1 to the current value of the variable n, and n-- subtracts 1 from it. For example, the code int n = 12; n++; changes n to 13. Since these operators change the value of a variable, they cannot be applied to numbers themselves. For example, 4++ is not a legal statement. There are two forms of these operators; you’ve just seen the postfix form of the operator that is placed after the operand. There is also a prefix form, ++n. Both change the value of the variable by 1. The difference between the two appears only when they are used inside expressions. The prefix form does the addition first; the postfix form evaluates to the old value of the variable. int int int int m n a b = = = = 7; 7; 2 * ++m; // now a is 16, m is 8 2 * n++; // now b is 14, n is 8 We recommend against using ++ inside expressions because this often leads to confusing code and annoying bugs. From the Library of Hristo Dimov Hristov 3.5 Operators 59 3.5.7 Relational and boolean Operators Java has the full complement of relational operators. To test for equality, use a double equal sign, ==. For example, the value of 3 == 7 is false. Use a != for inequality. For example, the value of 3 != 7 is true. Finally, you have the usual < (less than), > (greater than), <= (less than or equal), and >= (greater than or equal) operators. Java, following C++, uses && for the logical “and” operator and || for the logical “or” operator. As you can easily remember from the != operator, the exclamation point ! is the logical negation operator. The && and || operators are evaluated in “short circuit” fashion: The second argument is not evaluated if the first argument already determines the value. If you combine two expressions with the && operator, expression1 && expression2 and the truth value of the first expression has been determined to be false, then it is impossible for the result to be true. Thus, the value for the second expression is not calculated. This behavior can be exploited to avoid errors. For example, in the expression x != 0 && 1 / x > x + y // no division by 0 the second part is never evaluated if x equals zero. Thus, 1 / x is not computed if x is zero, and no divide-by-zero error can occur. Similarly, the value of expression1 || expression2 is automatically true if the first expression is true, without evaluating the second expression. Finally, Java supports the ternary ?: operator that is occasionally useful. The expression condition ? expression1 : expression2 evaluates to the first expression if the condition is true, to the second expression otherwise. For example, x < y ? x : y gives the smaller of x and y. From the Library of Hristo Dimov Hristov 60 Chapter 3 Fundamental Programming Structures in Java 3.5.8 Bitwise Operators For any of the integer types, you have operators that can work directly with the bits that make up the integers. This means that you can use masking techniques to get at individual bits in a number. The bitwise operators are & (“and”) | (“or”) ^ (“xor”) ~ (“not”) These operators work on bit patterns. For example, if n is an integer variable, then int fourthBitFromRight = (n & 0b1000) / 0b1000; gives you a 1 if the fourth bit from the right in the binary representation of n is 1, and 0 otherwise. Using & with the appropriate power of 2 lets you mask out all but a single bit. NOTE: When applied to boolean values, the & and | operators yield a boolean value. These operators are similar to the && and || operators, except that the & and | operators are not evaluated in “short circuit” fashion—that is, both arguments are evaluated before the result is computed. There are also >> and << operators which shift a bit pattern right or left. These operators are convenient when you need to build up bit patterns to do bit masking: int fourthBitFromRight = (n & (1 << 3)) >> 3; Finally, a >>> operator fills the top bits with zero, unlike >> which extends the sign bit into the top bits. There is no <<< operator. CAUTION: The right-hand argument of the shift operators is reduced modulo 32 (unless the left-hand argument is a long, in which case the right-hand argument is reduced modulo 64). For example, the value of 1 << 35 is the same as 1 << 3 or 8. C++ NOTE: In C/C++, there is no guarantee as to whether >> performs an arithmetic shift (extending the sign bit) or a logical shift (filling in with zeroes). Implementors are free to choose whichever is more efficient. That means the C/C++ >> operator may yield implementation-dependent results for negative numbers. Java removes that uncertainty. From the Library of Hristo Dimov Hristov 3.5 Operators 61 3.5.9 Parentheses and Operator Hierarchy Table 3.4 shows the precedence of operators. If no parentheses are used, operations are performed in the hierarchical order indicated. Operators on the same level are processed from left to right, except for those that are rightassociative, as indicated in the table. For example, && has a higher precedence than ||, so the expression a && b || c means (a && b) || c Table 3.4 Operator Precedence Operators Associativity [] . () (method call) Left to right ! ~ ++ -- + (unary) - (unary) () (cast) new Right to left * / % Left to right + - Left to right << >> >>> Left to right < <= > >= instanceof Left to right == != Left to right & Left to right ^ Left to right | Left to right && Left to right || Left to right ?: Right to left = += -= *= /= %= &= |= ^= <<= >>= >>>= Right to left Since += associates right to left, the expression a += b += c means a += (b += c) From the Library of Hristo Dimov Hristov 62 Chapter 3 Fundamental Programming Structures in Java That is, the value of b += c (which is the value of b after the addition) is added to a. C++ NOTE: Unlike C or C++, Java does not have a comma operator. However, you can use a comma-separated list of expressions in the first and third slot of a for statement. 3.6 Strings Conceptually, Java strings are sequences of Unicode characters. For example, the string "Java\u2122" consists of the five Unicode characters J, a, v, a, and ™. Java does not have a built-in string type. Instead, the standard Java library contains a predefined class called, naturally enough, String. Each quoted string is an instance of the String class: String e = ""; // an empty string String greeting = "Hello"; 3.6.1 Substrings You can extract a substring from a larger string with the substring method of the String class. For example, String greeting = "Hello"; String s = greeting.substring(0, 3); creates a string consisting of the characters "Hel". NOTE: Like C and C++, Java counts code units and code points in strings starting with 0. The second parameter of substring is the first position that you do not want to copy. In our case, we want to copy positions 0, 1, and 2 (from position 0 to position 2 inclusive). As substring counts it, this means from position 0 inclusive to position 3 exclusive. There is one advantage to the way substring works: Computing the length of the substring is easy. The string s.substring(a, b) always has length b − a. For example, the substring "Hel" has length 3 − 0 = 3. From the Library of Hristo Dimov Hristov 3.6 Strings 63 3.6.2 Concatenation Java, like most programming languages, allows you to use + to join (concatenate) two strings. String expletive = "Expletive"; String PG13 = "deleted"; String message = expletive + PG13; The preceding code sets the variable message to the string "Expletivedeleted". (Note the lack of a space between the words: The + operator joins two strings in the order received, exactly as they are given.) When you concatenate a string with a value that is not a string, the latter is converted to a string. (As you will see in Chapter 5, every Java object can be converted to a string.) For example, int age = 13; String rating = "PG" + age; sets rating to the string "PG13". This feature is commonly used in output statements. For example, System.out.println("The answer is " + answer); is perfectly acceptable and prints what you would expect (and with correct spacing because of the space after the word is). If you need to put multiple strings together, separated by a delimiter, use the static join method: String all = String.join(" / ", "S", "M", "L", "XL"); // all is the string "S / M / L / XL" As of Java 11, there is a repeat method: String repeated = "Java".repeat(3); // repeated is "JavaJavaJava" 3.6.3 Strings Are Immutable The String class gives no methods that let you change a character in an existing string. If you want to turn greeting into "Help!", you cannot directly change the last positions of greeting into 'p' and '!'. If you are a C programmer, this can make you feel pretty helpless. How are we going to modify the string? In Java, it is quite easy: Concatenate the substring that you want to keep with the characters that you want to replace. greeting = greeting.substring(0, 3) + "p!"; From the Library of Hristo Dimov Hristov 64 Chapter 3 Fundamental Programming Structures in Java This declaration changes the current value of the greeting variable to "Help!". Since you cannot change the individual characters in a Java string, the documentation refers to the objects of the String class as immutable. Just as the number 3 is always 3, the string "Hello" will always contain the code-unit sequence for the characters H, e, l, l, o. You cannot change these values. Yet you can, as you just saw, change the contents of the string variable greeting and make it refer to a different string, just as you can make a numeric variable currently holding the value 3 hold the value 4. Isn’t that a lot less efficient? It would seem simpler to change the code units than to build up a whole new string from scratch. Well, yes and no. Indeed, it isn’t efficient to generate a new string that holds the concatenation of "Hel" and "p!". But immutable strings have one great advantage: The compiler can arrange that strings are shared. To understand how this works, think of the various strings as sitting in a common pool. String variables then point to locations in the pool. If you copy a string variable, both the original and the copy share the same characters. Overall, the designers of Java decided that the efficiency of sharing outweighs the inefficiency of string editing by extracting substrings and concatenating. Look at your own programs; we suspect that most of the time, you don’t change strings—you just compare them. (There is one common exception— assembling strings from individual characters or from shorter strings that come from the keyboard or a file. For these situations, Java provides a separate class that we describe in Section 3.6.9, “Building Strings,” on p. 74.) C++ NOTE: C programmers are generally bewildered when they see Java strings for the first time because they think of strings as arrays of characters: char greeting[] = "Hello"; That is a wrong analogy: A Java string is roughly analogous to a char* pointer, char* greeting = "Hello"; When you replace greeting with another string, the Java code does roughly the following: char* temp = malloc(6); strncpy(temp, greeting, 3); strncpy(temp + 3, "p!", 3); greeting = temp; Sure, now greeting points to the string "Help!". And even the most hardened C programmer must admit that the Java syntax is more pleasant than a sequence of strncpy calls. But what if we make another assignment to greeting? From the Library of Hristo Dimov Hristov 3.6 Strings 65 greeting = "Howdy"; Don’t we have a memory leak? After all, the original string was allocated on the heap. Fortunately, Java does automatic garbage collection. If a block of memory is no longer needed, it will eventually be recycled. If you are a C++ programmer and use the string class defined by ANSI C++, you will be much more comfortable with the Java String type. C++ string objects also perform automatic allocation and deallocation of memory. The memory management is performed explicitly by constructors, assignment operators, and destructors. However, C++ strings are mutable—you can modify individual characters in a string. 3.6.4 Testing Strings for Equality To test whether two strings are equal, use the equals method. The expression s.equals(t) returns true if the strings s and t are equal, false otherwise. Note that s and t can be string variables or string literals. For example, the expression "Hello".equals(greeting) is perfectly legal. To test whether two strings are identical except for the upper/lowercase letter distinction, use the equalsIgnoreCase method. "Hello".equalsIgnoreCase("hello") Do not use the == operator to test whether two strings are equal! It only determines whether or not the strings are stored in the same location. Sure, if strings are in the same location, they must be equal. But it is entirely possible to store multiple copies of identical strings in different places. String greeting = "Hello"; // initialize greeting to a string if (greeting == "Hello") . . . // probably true if (greeting.substring(0, 3) == "Hel") . . . // probably false If the virtual machine always arranges for equal strings to be shared, then you could use the == operator for testing equality. But only string literals are shared, not strings that are the result of operations like + or substring. Therefore, never use == to compare strings lest you end up with a program with the worst kind of bug—an intermittent one that seems to occur randomly. From the Library of Hristo Dimov Hristov 66 Chapter 3 Fundamental Programming Structures in Java C++ NOTE: If you are used to the C++ string class, you have to be particularly careful about equality testing. The C++ string class does overload the == operator to test for equality of the string contents. It is perhaps unfortunate that Java goes out of its way to give strings the same “look and feel” as numeric values but then makes strings behave like pointers for equality testing. The language designers could have redefined == for strings, just as they made a special arrangement for +. Oh well, every language has its share of inconsistencies. C programmers never use == to compare strings but use strcmp instead. The Java method compareTo is the exact analog of strcmp. You can use if (greeting.compareTo("Hello") == 0) . . . but it seems clearer to use equals instead. 3.6.5 Empty and Null Strings The empty string "" is a string of length 0. You can test whether a string is empty by calling if (str.length() == 0) or if (str.equals("")) An empty string is a Java object which holds the string length (namely, 0) and an empty contents. However, a String variable can also hold a special value, called null, that indicates that no object is currently associated with the variable. (See Chapter 4 for more information about null.) To test whether a string is null, use if (str == null) Sometimes, you need to test that a string is neither null nor empty. Then use if (str != null && str.length() != 0) You need to test that str is not null first. As you will see in Chapter 4, it is an error to invoke a method on a null value. 3.6.6 Code Points and Code Units Java strings are sequences of char values. As we discussed in Section 3.3.3, “The char Type,” on p. 46, the char data type is a code unit for representing Unicode code points in the UTF-16 encoding. The most commonly used Unicode characters can be represented with a single code unit. The supplementary characters require a pair of code units. From the Library of Hristo Dimov Hristov 3.6 Strings 67 The length method yields the number of code units required for a given string in the UTF-16 encoding. For example: String greeting = "Hello"; int n = greeting.length(); // is 5 To get the true length—that is, the number of code points—call int cpCount = greeting.codePointCount(0, greeting.length()); The call s.charAt(n) returns the code unit at position n, where n is between 0 and s.length() – 1. For example: char first = greeting.charAt(0); // first is 'H' char last = greeting.charAt(4); // last is 'o' To get at the ith code point, use the statements int index = greeting.offsetByCodePoints(0, i); int cp = greeting.codePointAt(index); Why are we making a fuss about code units? Consider the sentence is the set of octonions. The character Calling (U+1D546) requires two code units in the UTF-16 encoding. char ch = sentence.charAt(1) doesn’t return a space but the second code unit of . To avoid this problem, you should not use the char type. It is too low-level. NOTE: Don’t think that you can ignore exotic characters with code units above U+FFFF. Your emoji-loving users may put characters such as (U+1F37A, beer mug) into strings. If your code traverses a string, and you want to look at each code point in turn, you can use these statements: int cp = sentence.codePointAt(i); if (Character.isSupplementaryCodePoint(cp)) i += 2; else i++; You can move backwards with the following statements: i--; if (Character.isSurrogate(sentence.charAt(i))) i--; int cp = sentence.codePointAt(i); From the Library of Hristo Dimov Hristov 68 Chapter 3 Fundamental Programming Structures in Java Obviously, that is quite painful. An easier way is to use the codePoints method that yields a “stream” of int values, one for each code point. (We will discuss streams in Chapter 2 of Volume II.) You can just turn the stream into an array (see Section 3.10, “Arrays,” on p. 108) and traverse that. int[] codePoints = str.codePoints().toArray(); Conversely, to turn an array of code points to a string, use a constructor. (We discuss constructors and the new operator in detail in Chapter 4.) String str = new String(codePoints, 0, codePoints.length); NOTE: The virtual machine does not have to implement strings as sequences of code units. In Java 9, strings that hold only single-byte code units use a byte array, and all others a char array. 3.6.7 The String API The String class in Java contains more than 50 methods. A surprisingly large number of them are sufficiently useful that we can imagine using them frequently. The following API note summarizes the ones we found most useful. These API notes, found throughout the book, will help you understand the Java Application Programming Interface (API). Each API note starts with the name of a class, such as java.lang.String. (The significance of the so-called package name java.lang is explained in Chapter 4.) The class name is followed by the names, explanations, and parameter descriptions of one or more methods. We typically do not list all methods of a particular class but select those that are most commonly used and describe them in a concise form. For a full listing, consult the online documentation (see Section 3.6.8, “Reading the Online API Documentation,” on p. 71). We also list the version number in which a particular class was introduced. If a method has been added later, it has a separate version number. java.lang.String 1.0 • char charAt(int index) returns the code unit at the specified location. You probably don’t want to call this method unless you are interested in low-level code units. (Continues) From the Library of Hristo Dimov Hristov 3.6 Strings 69 java.lang.String 1.0 (Continued) • int codePointAt(int index) 5 returns the code point that starts at the specified location. • int offsetByCodePoints(int startIndex, int cpCount) 5 returns the index of the code point that is cpCount code points away from the code point at startIndex. • int compareTo(String other) returns a negative value if the string comes before other in dictionary order, a positive value if the string comes after other in dictionary order, or 0 if the strings are equal. • IntStream codePoints() 8 returns the code points of this string as a stream. Call toArray to put them in an array. • new String(int[] codePoints, int offset, int count) 5 constructs a string with the count code points in the array starting at offset. • boolean isEmpty() boolean isBlank() 11 returns true if the string is empty or consists of whitespace. • boolean equals(Object other) returns true if the string equals other. • boolean equalsIgnoreCase(String other) returns true if the string equals other, except for upper/lowercase distinction. • boolean startsWith(String prefix) • boolean endsWith(String suffix) returns true if the string starts with prefix or ends with suffix. • • • • int int int int indexOf(String str) indexOf(String str, int fromIndex) indexOf(int cp) indexOf(int cp, int fromIndex) returns the start of the first substring equal to the string str or the code point cp, starting at index 0 or at fromIndex, or -1 if str does not occur in this string. (Continues) From the Library of Hristo Dimov Hristov 70 Chapter 3 Fundamental Programming Structures in Java java.lang.String 1.0 (Continued) • • • • int int int int lastIndexOf(String str) lastIndexOf(String str, int fromIndex) lastindexOf(int cp) lastindexOf(int cp, int fromIndex) returns the start of the last substring equal to the string str or the code point cp, starting at the end of the string or at fromIndex. • int length() returns the number of code units of the string. • int codePointCount(int startIndex, int endIndex) 5 returns the number of code points between startIndex and endIndex – 1. • String replace(CharSequence oldString, CharSequence newString) returns a new string that is obtained by replacing all substrings matching oldString in the string with the string newString. You can supply String or StringBuilder objects for the CharSequence parameters. • String substring(int beginIndex) • String substring(int beginIndex, int endIndex) returns a new string consisting of all code units from beginIndex until the end of the string or until endIndex – 1. • String toLowerCase() • String toUpperCase() returns a new string containing all characters in the original string, with uppercase characters converted to lowercase, or lowercase characters converted to uppercase. • String trim() String strip() 11 returns a new string by eliminating all leading and trailing characters that are ≤ U+0020 (trim) or whitespace (strip) in the original string. • String join(CharSequence delimiter, CharSequence... elements) 8 returns a new string joining all elements with the given delimiter. • String repeat(int count) 11 returns a string that repeats this string count times. From the Library of Hristo Dimov Hristov 3.6 Strings 71 NOTE: In the API notes, there are a few parameters of type CharSequence. This is an interface type to which all strings belong. You will learn about interface types in Chapter 6. For now, you just need to know that you can pass arguments of type String whenever you see a CharSequence parameter. 3.6.8 Reading the Online API Documentation As you just saw, the String class has lots of methods. Furthermore, there are thousands of classes in the standard libraries, with many more methods. It is plainly impossible to remember all useful classes and methods. Therefore, it is essential that you become familiar with the online API documentation that lets you look up all classes and methods in the standard library. You can download the API documentation from Oracle and save it locally, or you can point your browser to https://docs.oracle.com/en/java/javase/11/docs/api. Figure 3.2 The Java API documentation From the Library of Hristo Dimov Hristov 72 Chapter 3 Fundamental Programming Structures in Java As of Java 9, the API documentation has a search box (see Figure 3.2). Older versions have frames with lists of packages and classes. You can still get those lists by clicking on the Frames menu item. For example, to get more information on the methods of the String class, type “String” into the search box and select the type java.lang.String, or locate the link in the frame with class names and click it. You get the class description, as shown in Figure 3.3. Figure 3.3 Class description for the String class When you scroll down, you reach a summary of all methods, sorted in alphabetical order (see Figure 3.4). Click on any method name for a detailed description of that method (see Figure 3.5). For example, if you click on the compareToIgnoreCase link, you’ll get the description of the compareToIgnoreCase method. TIP: If you have not already done so, download the JDK documentation, as described in Chapter 2. Bookmark the jdk-11-docs/index.html page in your browser right now. From the Library of Hristo Dimov Hristov 3.6 Strings 73 Figure 3.4 Method summary of the String class Figure 3.5 Detailed description of a String method From the Library of Hristo Dimov Hristov 74 Chapter 3 Fundamental Programming Structures in Java 3.6.9 Building Strings Occasionally, you need to build up strings from shorter strings, such as keystrokes or words from a file. It would be inefficient to use string concatenation for this purpose. Every time you concatenate strings, a new String object is constructed. This is time consuming and wastes memory. Using the StringBuilder class avoids this problem. Follow these steps if you need to build a string from many small pieces. First, construct an empty string builder: StringBuilder builder = new StringBuilder(); Each time you need to add another part, call the append method. builder.append(ch); // appends a single character builder.append(str); // appends a string When you are done building the string, call the toString method. You will get a String object with the character sequence contained in the builder. String completedString = builder.toString(); NOTE: The StringBuilder class was introduced in Java 5. Its predecessor, StringBuffer, is slightly less efficient, but it allows multiple threads to add or remove characters. If all string editing happens in a single thread (which is usually the case), you should use StringBuilder instead. The APIs of both classes are identical. The following API notes contain the most important methods for the StringBuilder class. java.lang.StringBuilder 5 • StringBuilder() constructs an empty string builder. • int length() returns the number of code units of the builder or buffer. • StringBuilder append(String str) appends a string and returns this. (Continues) From the Library of Hristo Dimov Hristov 3.7 Input and Output 75 java.lang.StringBuilder 5 (Continued) • StringBuilder append(char c) appends a code unit and returns this. • StringBuilder appendCodePoint(int cp) appends a code point, converting it into one or two code units, and returns this. • void setCharAt(int i, char c) sets the ith code unit to c. • StringBuilder insert(int offset, String str) inserts a string at position offset and returns this. • StringBuilder insert(int offset, char c) inserts a code unit at position offset and returns this. • StringBuilder delete(int startIndex, int endIndex) deletes the code units with offsets startIndex to endIndex – 1 and returns this. • String toString() returns a string with the same data as the builder or buffer contents. 3.7 Input and Output To make our example programs more interesting, we want to accept input and properly format the program output. Of course, modern programs use a GUI for collecting user input. However, programming such an interface requires more tools and techniques than we have at our disposal at this time. Our first order of business is to become more familiar with the Java programming language, so we use the humble console for input and output. 3.7.1 Reading Input You saw that it is easy to print output to the “standard output stream” (that is, the console window) just by calling System.out.println. Reading from the “standard input stream” System.in isn’t quite as simple. To read console input, you first construct a Scanner that is attached to System.in: Scanner in = new Scanner(System.in); (We discuss constructors and the new operator in detail in Chapter 4.) From the Library of Hristo Dimov Hristov 76 Chapter 3 Fundamental Programming Structures in Java Now you can use the various methods of the Scanner class to read input. For example, the nextLine method reads a line of input. System.out.print("What is your name? "); String name = in.nextLine(); Here, we use the nextLine method because the input might contain spaces. To read a single word (delimited by whitespace), call String firstName = in.next(); To read an integer, use the nextInt method. System.out.print("How old are you? "); int age = in.nextInt(); Similarly, the nextDouble method reads the next floating-point number. The program in Listing 3.2 asks for the user’s name and age and then prints a message like Hello, Cay. Next year, you'll be 57 Finally, note the line import java.util.*; at the beginning of the program. The Scanner class is defined in the java.util package. Whenever you use a class that is not defined in the basic java.lang package, you need to use an import directive. We look at packages and import directives in more detail in Chapter 4. Listing 3.2 InputTest/InputTest.java 1 import java.util.*; 2 3 4 5 6 7 8 9 10 11 12 /** * This program demonstrates console input. * @version 1.10 2004-02-10 * @author Cay Horstmann */ public class InputTest { public static void main(String[] args) { Scanner in = new Scanner(System.in); 13 14 15 16 // get first input System.out.print("What is your name? "); String name = in.nextLine(); From the Library of Hristo Dimov Hristov 3.7 Input and Output 77 17 // get second input System.out.print("How old are you? "); int age = in.nextInt(); 18 19 20 21 // display output on console System.out.println("Hello, " + name + ". Next year, you'll be " + (age + 1)); 22 23 } 24 25 } NOTE: The Scanner class is not suitable for reading a password from a console since the input is plainly visible to anyone. Java 6 introduces a Console class specifically for this purpose. To read a password, use the following code: Console cons = System.console(); String username = cons.readLine("User name: "); char[] passwd = cons.readPassword("Password: "); For security reasons, the password is returned in an array of characters rather than a string. After you are done processing the password, you should immediately overwrite the array elements with a filler value. (Array processing is discussed in Section 3.10, “Arrays,” on p. 108.) Input processing with a Console object is not as convenient as with a Scanner. You must read the input a line at a time. There are no methods for reading individual words or numbers. java.util.Scanner 5 • Scanner(InputStream in) constructs a Scanner object from the given input stream. • String nextLine() reads the next line of input. • String next() reads the next word of input (delimited by whitespace). • int nextInt() • double nextDouble() reads and converts the next character sequence that represents an integer or floating-point number. • boolean hasNext() tests whether there is another word in the input. (Continues) From the Library of Hristo Dimov Hristov 78 Chapter 3 Fundamental Programming Structures in Java java.util.Scanner 5 (Continued) • boolean hasNextInt() • boolean hasNextDouble() tests whether the next character sequence represents an integer or floatingpoint number. java.lang.System 1.0 • static Console console() 6 returns a Console object for interacting with the user through a console window if such interaction is possible, null otherwise. A Console object is available for any program that is launched in a console window. Otherwise, the availability is system-dependent. java.io.Console 6 • static char[] readPassword(String prompt, Object... args) • static String readLine(String prompt, Object... args) displays the prompt and reads the user input until the end of the input line. The args parameters can be used to supply formatting arguments, as described in the next section. 3.7.2 Formatting Output You can print a number x to the console with the statement System.out.print(x). That command will print x with the maximum number of nonzero digits for that type. For example, double x = 10000.0 / 3.0; System.out.print(x); prints 3333.3333333333335 That is a problem if you want to display, for example, dollars and cents. In early versions of Java, formatting numbers was a bit of a hassle. Fortunately, Java 5 brought back the venerable printf method from the C library. For example, the call System.out.printf("%8.2f", x); From the Library of Hristo Dimov Hristov 3.7 Input and Output 79 prints x with a field width of 8 characters and a precision of 2 characters. That is, the printout contains a leading space and the seven characters 3333.33 You can supply multiple parameters to printf. For example: System.out.printf("Hello, %s. Next year, you'll be %d", name, age); Each of the format specifiers that start with a % character is replaced with the corresponding argument. The conversion character that ends a format specifier indicates the type of the value to be formatted: f is a floating-point number, s a string, and d a decimal integer. Table 3.5 shows all conversion characters. Table 3.5 Conversions for printf Conversion Character Type Example d Decimal integer 159 x Hexadecimal integer 9f o Octal integer 237 f Fixed-point floating-point 15.9 e Exponential floating-point 1.59e+01 g General floating-point (the shorter of e and f) — a Hexadecimal floating-point 0x1.fccdp3 s String Hello c Character H b boolean true h Hash code 42628b2 tx or Tx Date and time (T forces uppercase) Obsolete, use the java.time classes instead—see Chapter 6 of Volume II % The percent symbol % n The platform-dependent line separator — From the Library of Hristo Dimov Hristov 80 Chapter 3 Fundamental Programming Structures in Java In addition, you can specify flags that control the appearance of the formatted output. Table 3.6 shows all flags. For example, the comma flag adds group separators. That is, System.out.printf("%,.2f", 10000.0 / 3.0); prints 3,333.33 You can use multiple flags, for example "%,(.2f" to use group separators and enclose negative numbers in parentheses. Table 3.6 Flags for printf Flag Purpose Example + Prints sign for positive and negative numbers. +3333.33 space Adds a space before positive numbers. | 3333.33| 0 Adds leading zeroes. 003333.33 - Left-justifies field. |3333.33 | ( Encloses negative numbers in parentheses. (3333.33) , Adds group separators. 3,333.33 # (for f format) Always includes a decimal point. 3,333. # (for x or o format) Adds 0x or 0 prefix. 0xcafe $ Specifies the index of the argument to be formatted; for example, %1$d %1$x prints the first argument in decimal and hexadecimal. 159 9F < Formats the same value as the previous specification; for example, %d %<x prints the same number in decimal and hexadecimal. 159 9F NOTE: You can use the s conversion to format arbitrary objects. If an arbitrary object implements the Formattable interface, the object’s formatTo method is invoked. Otherwise, the toString method is invoked to turn the object into a string. We discuss the toString method in Chapter 5 and interfaces in Chapter 6. You can use the static String.format method to create a formatted string without printing it: From the Library of Hristo Dimov Hristov 3.7 Input and Output 81 String message = String.format("Hello, %s. Next year, you'll be %d", name, age); In the interest of completeness, we briefly discuss the date and time formatting options of the printf method. For new code, you should use the methods of the java.time package described in Chapter 6 of Volume II. But you may encounter the Date class and the associated formatting options in legacy code. The format consists of two letters, starting with t and ending in one of the letters of Table 3.7; for example, System.out.printf("%tc", new Date()); prints the current date and time in the format Mon Feb 09 18:05:19 PST 2015 Table 3.7 Date and Time Conversion Characters Conversion Character Type Example c Complete date and time Mon Feb 09 18:05:19 PST 2015 F ISO 8601 date 2015-02-09 D U.S. formatted date (month/day/year) 02/09/2015 T 24-hour time 18:05:19 r 12-hour time 06:05:19 pm R 24-hour time, no seconds 18:05 Y Four-digit year (with leading zeroes) 2015 y Last two digits of the year (with leading zeroes) 15 C First two digits of the year (with leading zeroes) 20 B Full month name February b or h Abbreviated month name Feb m Two-digit month (with leading zeroes) 02 d Two-digit day (with leading zeroes) 09 e Two-digit day (without leading zeroes) 9 A Full weekday name Monday (Continues) From the Library of Hristo Dimov Hristov 82 Chapter 3 Fundamental Programming Structures in Java Table 3.7 (Continued) Conversion Character Type Example a Abbreviated weekday name Mon j Three-digit day of year (with leading zeroes), between 001 and 366 069 H Two-digit hour (with leading zeroes), between 00 and 23 18 k Two-digit hour (without leading zeroes), between 0 and 23 18 I Two-digit hour (with leading zeroes), between 01 and 12 06 l Two-digit hour (without leading zeroes), between 1 and 12 6 M Two-digit minutes (with leading zeroes) 05 S Two-digit seconds (with leading zeroes) 19 L Three-digit milliseconds (with leading zeroes) 047 N Nine-digit nanoseconds (with leading zeroes) 047000000 p Morning or afternoon marker pm z RFC 822 numeric offset from GMT -0800 Z Time zone PST s Seconds since 1970–01–01 00:00:00 GMT 1078884319 Q Milliseconds since 1970–01–01 00:00:00 GMT 1078884319047 As you can see in Table 3.7, some of the formats yield only a part of a given date—for example, just the day or just the month. It would be a bit silly if you had to supply the date multiple times to format each part. For that reason, a format string can indicate the index of the argument to be formatted. The index must immediately follow the %, and it must be terminated by a $. For example, System.out.printf("%1$s %2$tB %2$te, %2$tY", "Due date:", new Date()); From the Library of Hristo Dimov Hristov 3.7 Input and Output 83 prints Due date: February 9, 2015 Alternatively, you can use the < flag. It indicates that the same argument as in the preceding format specification should be used again. That is, the statement System.out.printf("%s %tB %<te, %<tY", "Due date:", new Date()); yields the same output as the preceding statement. CAUTION: Argument index values start with 1, not with 0: %1$. . . formats the first argument. This avoids confusion with the 0 flag. You have now seen all features of the printf method. Figure 3.6 shows a syntax diagram for format specifiers. Figure 3.6 Format specifier syntax NOTE: The formatting of numbers and dates is locale-specific. For example, in Germany, the group separator is a period, not a comma, and Monday is formatted as Montag. Chapter 7 of Volume II shows how to control the international behavior of your applications. 3.7.3 File Input and Output To read from a file, construct a Scanner object like this: Scanner in = new Scanner(Path.of("myfile.txt"), StandardCharsets.UTF_8); If the file name contains backslashes, remember to escape each of them with an additional backslash: "c:\\mydirectory\\myfile.txt". From the Library of Hristo Dimov Hristov 84 Chapter 3 Fundamental Programming Structures in Java NOTE: Here, we specify the UTF-8 character encoding, which is common (but not universal) for files on the Internet. You need to know the character encoding when you read a text file (see Volume II, Chapter 2 for more information). If you omit the character encoding, then the “default encoding” of the computer running the Java program is used. That is not a good idea—the program might act differently depending on where it is run. Now you can read from the file, using any of the Scanner methods that we already described. To write to a file, construct a PrintWriter object. In the constructor, supply the file name and the character encoding: PrintWriter out = new PrintWriter("myfile.txt", StandardCharsets.UTF_8); If the file does not exist, it is created. You can use the print, println, and printf commands as you did when printing to System.out. CAUTION: You can construct a Scanner with a string parameter, but the scanner interprets the string as data, not a file name. For example, if you call Scanner in = new Scanner("myfile.txt"); // ERROR? then the scanner will see ten characters of data: 'm', 'y', 'f', and so on. That is probably not what was intended in this case. NOTE: When you specify a relative file name, such as "myfile.txt", "mydirectory/myfile.txt", or "../myfile.txt", the file is located relative to the directory in which the Java virtual machine was started. If you launched your program from a command shell, by executing java MyProg then the starting directory is the current directory of the command shell. However, if you use an integrated development environment, it controls the starting directory. You can find the directory location with this call: String dir = System.getProperty("user.dir"); If you run into grief with locating files, consider using absolute path names such as "c:\\mydirectory\\myfile.txt" or "/home/me/mydirectory/myfile.txt". From the Library of Hristo Dimov Hristov 3.7 Input and Output 85 As you saw, you can access files just as easily as you can use System.in and System.out. There is just one catch: If you construct a Scanner with a file that does not exist or a PrintWriter with a file name that cannot be created, an exception occurs. The Java compiler considers these exceptions to be more serious than a “divide by zero” exception, for example. In Chapter 7, you will learn various ways of handling exceptions. For now, you should simply tell the compiler that you are aware of the possibility of an “input/output” exception. You do this by tagging the main method with a throws clause, like this: public static void main(String[] args) throws IOException { Scanner in = new Scanner(Path.of("myfile.txt"), StandardCharsets.UTF_8); . . . } You have now seen how to read and write files that contain textual data. For more advanced topics, such as dealing with different character encodings, processing binary data, reading directories, and writing zip files, turn to Chapter 2 of Volume II. NOTE: When you launch a program from a command shell, you can use the redirection syntax of your shell and attach any file to System.in and System.out: java MyProg < myfile.txt > output.txt Then, you need not worry about handling the IOException. java.util.Scanner 5 • Scanner(Path p, String encoding) constructs a Scanner that reads data from the given path, using the given character encoding. • Scanner(String data) constructs a Scanner that reads data from the given string. java.io.PrintWriter 1.1 • PrintWriter(String fileName) constructs a PrintWriter that writes data to the file with the given file name. From the Library of Hristo Dimov Hristov 86 Chapter 3 Fundamental Programming Structures in Java java.nio.file.Path • static Path of(String pathname) 11 constructs a Path from the given path name. 3.8 Control Flow Java, like any programming language, supports both conditional statements and loops to determine control flow. We will start with the conditional statements, then move on to loops, to end with the somewhat cumbersome switch statement that you can use to test for many values of a single expression. C++ NOTE: The Java control flow constructs are identical to those in C and C++, with a few exceptions. There is no goto, but there is a “labeled” version of break that you can use to break out of a nested loop (where, in C, you perhaps would have used a goto). Finally, there is a variant of the for loop that is similar to the range-based for loop in C++ and the foreach loop in C#. 3.8.1 Block Scope Before learning about control structures, you need to know more about blocks. A block, or compound statement, consists of a number of Java statements, surrounded by a pair of braces. Blocks define the scope of your variables. A block can be nested inside another block. Here is a block that is nested inside the block of the main method: public static void main(String[] args) { int n; . . . { int k; . . . } // k is only defined up to here } You may not declare identically named variables in two nested blocks. For example, the following is an error and will not compile: public static void main(String[] args) { int n; . . . From the Library of Hristo Dimov Hristov 3.8 Control Flow 87 { int k; int n; // ERROR--can't redefine n in inner block . . . } } C++ NOTE: In C++, it is possible to redefine a variable inside a nested block. The inner definition then shadows the outer one. This can be a source of programming errors; hence, Java does not allow it. 3.8.2 Conditional Statements The conditional statement in Java has the form if (condition) statement The condition must be surrounded by parentheses. In Java, as in most programming languages, you will often want to execute multiple statements when a single condition is true. In this case, use a block statement that takes the form { statement1 statement2 . . . } For example: if (yourSales >= target) { performance = "Satisfactory"; bonus = 100; } In this code all the statements surrounded by the braces will be executed when yourSales is greater than or equal to target (see Figure 3.7). NOTE: A block (sometimes called a compound statement) enables you to have more than one (simple) statement in any Java programming structure that otherwise allows for a single (simple) statement. The more general conditional in Java looks like this (see Figure 3.8): if (condition) statement1 else statement2 From the Library of Hristo Dimov Hristov 88 Chapter 3 Fundamental Programming Structures in Java Figure 3.7 Flowchart for the if statement For example: if (yourSales >= { performance = bonus = 100 + } else { performance = bonus = 0; } target) "Satisfactory"; 0.01 * (yourSales - target); "Unsatisfactory"; The else part is always optional. An else groups with the closest if. Thus, in the statement if (x <= 0) if (x == 0) sign = 0; else sign = -1; From the Library of Hristo Dimov Hristov 3.8 Control Flow 89 Figure 3.8 Flowchart for the if/else statement the else belongs to the second if. Of course, it is a good idea to use braces to clarify this code: if (x <= 0) { if (x == 0) sign = 0; else sign = -1; } Repeated if . . . else if . . . alternatives are common (see Figure 3.9). For example: if (yourSales >= 2 * target) { performance = "Excellent"; bonus = 1000; } else if (yourSales >= 1.5 * target) { performance = "Fine"; bonus = 500; From the Library of Hristo Dimov Hristov 90 Chapter 3 Fundamental Programming Structures in Java } else if (yourSales >= target) { performance = "Satisfactory"; bonus = 100; } else { System.out.println("You're fired"); } Figure 3.9 Flowchart for the if/else if (multiple branches) From the Library of Hristo Dimov Hristov 3.8 Control Flow 91 3.8.3 Loops The while loop executes a statement (which may be a block statement) while a condition is true. The general form is while (condition) statement The while loop will never execute if the condition is false at the outset (see Figure 3.10). Figure 3.10 Flowchart for the while statement From the Library of Hristo Dimov Hristov 92 Chapter 3 Fundamental Programming Structures in Java The program in Listing 3.3 determines how long it will take to save a specific amount of money for your well-earned retirement, assuming you deposit the same amount of money per year and the money earns a specified interest rate. In the example, we are incrementing a counter and updating the amount currently accumulated in the body of the loop until the total exceeds the targeted amount. while (balance < goal) { balance += payment; double interest = balance * interestRate / 100; balance += interest; years++; } System.out.println(years + " years."); (Don’t rely on this program to plan for your retirement. We left out a few niceties such as inflation and your life expectancy.) A while loop tests at the top. Therefore, the code in the block might never be executed. If you want to make sure a block is executed at least once, you need to move the test to the bottom, using the do/while loop. Its syntax looks like this: do statement while (condition); This loop executes the statement (which is typically a block) and only then tests the condition. If it’s true, it repeats the statement and retests the condition, and so on. The code in Listing 3.4 computes the new balance in your retirement account and then asks if you are ready to retire: do { balance += payment; double interest = balance * interestRate / 100; balance += interest; year++; // print current balance . . . // ask if ready to retire and get input . . . } while (input.equals("N")); As long as the user answers "N", the loop is repeated (see Figure 3.11). This program is a good example of a loop that needs to be entered at least once, because the user needs to see the balance before deciding whether it is sufficient for retirement. From the Library of Hristo Dimov Hristov 3.8 Control Flow 93 Figure 3.11 Flowchart for the do/while statement Listing 3.3 Retirement/Retirement.java 1 import java.util.*; 2 3 4 5 6 7 8 9 /** * This program demonstrates a <code>while</code> loop. * @version 1.20 2004-02-10 * @author Cay Horstmann */ public class Retirement { (Continues) From the Library of Hristo Dimov Hristov 94 Chapter 3 Fundamental Programming Structures in Java Listing 3.3 (Continued) public static void main(String[] args) { // read inputs Scanner in = new Scanner(System.in); 10 11 12 13 14 System.out.print("How much money do you need to retire? "); double goal = in.nextDouble(); 15 16 17 System.out.print("How much money will you contribute every year? "); double payment = in.nextDouble(); 18 19 20 System.out.print("Interest rate in %: "); double interestRate = in.nextDouble(); 21 22 23 double balance = 0; int years = 0; 24 25 26 // update account balance while goal isn't reached while (balance < goal) { // add this year's payment and interest balance += payment; double interest = balance * interestRate / 100; balance += interest; years++; } 27 28 29 30 31 32 33 34 35 36 System.out.println("You can retire in " + years + " years."); 37 } 38 39 } Listing 3.4 Retirement2/Retirement2.java 1 import java.util.*; 2 3 4 5 6 7 8 9 10 11 /** * This program demonstrates a <code>do/while</code> loop. * @version 1.20 2004-02-10 * @author Cay Horstmann */ public class Retirement2 { public static void main(String[] args) { From the Library of Hristo Dimov Hristov 3.8 Control Flow 95 Scanner in = new Scanner(System.in); 12 13 System.out.print("How much money will you contribute every year? "); double payment = in.nextDouble(); 14 15 16 System.out.print("Interest rate in %: "); double interestRate = in.nextDouble(); 17 18 19 double balance = 0; int year = 0; 20 21 22 String input; 23 24 // update account balance while user isn't ready to retire do { // add this year's payment and interest balance += payment; double interest = balance * interestRate / 100; balance += interest; 25 26 27 28 29 30 31 32 year++; 33 34 // print current balance System.out.printf("After year %d, your balance is %,.2f%n", year, balance); 35 36 37 // ask if ready to retire and get input System.out.print("Ready to retire? (Y/N) "); input = in.next(); 38 39 40 } while (input.equals("N")); 41 42 } 43 44 } 3.8.4 Determinate Loops The for loop is a general construct to support iteration controlled by a counter or similar variable that is updated after every iteration. As Figure 3.12 shows, the following loop prints the numbers from 1 to 10 on the screen: for (int i = 1; i <= 10; i++) System.out.println(i); The first slot of the for statement usually holds the counter initialization. The second slot gives the condition that will be tested before each new pass through the loop, and the third slot specifies how to update the counter. From the Library of Hristo Dimov Hristov 96 Chapter 3 Fundamental Programming Structures in Java Figure 3.12 Flowchart for the for statement Although Java, like C++, allows almost any expression in the various slots of a for loop, it is an unwritten rule of good taste that the three slots should only initialize, test, and update the same counter variable. One can write very obscure loops by disregarding this rule. Even within the bounds of good taste, much is possible. For example, you can have loops that count down: for (int i = 10; i > 0; i--) System.out.println("Counting down . . . " + i); System.out.println("Blastoff!"); From the Library of Hristo Dimov Hristov 3.8 Control Flow 97 CAUTION: Be careful with testing for equality of floating-point numbers in loops. A for loop like this one for (double x = 0; x != 10; x += 0.1) . . . might never end. Because of roundoff errors, the final value might not be reached exactly. In this example, x jumps from 9.99999999999998 to 10.09999999999998 because there is no exact binary representation for 0.1. When you declare a variable in the first slot of the for statement, the scope of that variable extends until the end of the body of the for loop. for (int i = 1; i <= 10; i++) { . . . } // i no longer defined here In particular, if you define a variable inside a for statement, you cannot use its value outside the loop. Therefore, if you wish to use the final value of a loop counter outside the for loop, be sure to declare it outside the loop header. int i; for (i = 1; i <= 10; i++) { . . . } // i is still defined here On the other hand, you can define variables with the same name in separate for loops: for (int i = 1; i <= 10; i++) { . . . } . . . for (int i = 11; i <= 20; i++) // OK to define another variable named i { . . . } A for loop is merely a convenient shortcut for a while loop. For example, for (int i = 10; i > 0; i--) System.out.println("Counting down . . . " + i); can be rewritten as From the Library of Hristo Dimov Hristov 98 Chapter 3 Fundamental Programming Structures in Java int i = 10; while (i > 0) { System.out.println("Counting down . . . " + i); i--; } Listing 3.5 shows a typical example of a for loop. The program computes the odds of winning a lottery. For example, if you must pick six numbers from the numbers 1 to 50 to win, then there are (50 × 49 × 48 × 47 × 46 × 45)/(1 × 2 × 3 × 4 × 5 × 6) possible outcomes, so your chance is 1 in 15,890,700. Good luck! In general, if you pick k numbers out of n, there are n × (n – 1) × (n – 2) × · · · × (n – k + 1) 1×2×3×4×···×k possible outcomes. The following for loop computes this value: int lotteryOdds = 1; for (int i = 1; i <= k; i++) lotteryOdds = lotteryOdds * (n - i + 1) / i; NOTE: See Section 3.10.3, “The ‘for each’ Loop,” on p. 110 for a description of the “generalized for loop” (also called “for each” loop) that was added to the Java language in Java 5. Listing 3.5 LotteryOdds/LotteryOdds.java 1 import java.util.*; 2 3 4 5 6 7 8 9 10 11 12 /** * This program demonstrates a <code>for</code> loop. * @version 1.20 2004-02-10 * @author Cay Horstmann */ public class LotteryOdds { public static void main(String[] args) { Scanner in = new Scanner(System.in); 13 14 15 System.out.print("How many numbers do you need to draw? "); int k = in.nextInt(); From the Library of Hristo Dimov Hristov 3.8 Control Flow 99 16 System.out.print("What is the highest number you can draw? "); int n = in.nextInt(); 17 18 19 /* * compute binomial coefficient n*(n-1)*(n-2)*...*(n-k+1)/(1*2*3*...*k) */ 20 21 22 23 int lotteryOdds = 1; for (int i = 1; i <= k; i++) lotteryOdds = lotteryOdds * (n - i + 1) / i; 24 25 26 27 System.out.println("Your odds are 1 in " + lotteryOdds + ". Good luck!"); 28 } 29 30 } 3.8.5 Multiple Selections–The switch Statement The if/else construct can be cumbersome when you have to deal with multiple selections with many alternatives. Java has a switch statement that is exactly like the switch statement in C and C++, warts and all. For example, if you set up a menu system with four alternatives like that in Figure 3.13, you could use code that looks like this: Scanner in = new Scanner(System.in); System.out.print("Select an option (1, 2, 3, 4) "); int choice = in.nextInt(); switch (choice) { case 1: . . . break; case 2: . . . break; case 3: . . . break; case 4: . . . break; default: // bad input . . . break; } From the Library of Hristo Dimov Hristov 100 Chapter 3 Fundamental Programming Structures in Java Figure 3.13 Flowchart for the switch statement Execution starts at the case label that matches the value on which the selection is performed and continues until the next break or the end of the switch. If none of the case labels match, then the default clause is executed, if it is present. From the Library of Hristo Dimov Hristov 3.8 Control Flow 101 CAUTION: It is possible for multiple alternatives to be triggered. If you forget to add a break at the end of an alternative, execution falls through to the next alternative! This behavior is plainly dangerous and a common cause for errors. For that reason, we never use the switch statement in our programs. If you like the switch statement better than we do, consider compiling your code with the -Xlint:fallthrough option, like this: javac -Xlint:fallthrough Test.java Then the compiler will issue a warning whenever an alternative does not end with a break statement. If you actually want to use the fallthrough behavior, tag the surrounding method with the annotation @SuppressWarnings("fallthrough"). Then no warnings will be generated for that method. (An annotation is a mechanism for supplying information to the compiler or a tool that processes Java source or class files. We discuss annotations in detail in Chapter 8 of Volume II.) A case label can be • A constant expression of type char, byte, short, or int • An enumerated constant • Starting with Java 7, a string literal For example, String input = . . .; switch (input.toLowerCase()) { case "yes": // OK since Java 7 . . . break; . . . } When you use the switch statement with enumerated constants, you need not supply the name of the enumeration in each label—it is deduced from the switch value. For example: Size sz = . . .; switch (sz) { case SMALL: // no need to use Size.SMALL . . . break; . . . } From the Library of Hristo Dimov Hristov 102 Chapter 3 Fundamental Programming Structures in Java 3.8.6 Statements That Break Control Flow Although the designers of Java kept goto as a reserved word, they decided not to include it in the language. In general, goto statements are considered poor style. Some programmers feel the anti-goto forces have gone too far (see, for example, the famous article of Donald Knuth called “Structured Programming with goto statements”). They argue that unrestricted use of goto is error-prone but that an occasional jump out of a loop is beneficial. The Java designers agreed and even added a new statement, the labeled break, to support this programming style. Let us first look at the unlabeled break statement. The same break statement that you use to exit a switch can also be used to break out of a loop. For example: while (years <= 100) { balance += payment; double interest = balance * interestRate / 100; balance += interest; if (balance >= goal) break; years++; } Now the loop is exited if either years > 100 occurs at the top of the loop or balance >= goal occurs in the middle of the loop. Of course, you could have computed the same value for years without a break, like this: while (years <= 100 && balance < goal) { balance += payment; double interest = balance * interestRate / 100; balance += interest; if (balance < goal) years++; } But note that the test balance < goal is repeated twice in this version. To avoid this repeated test, some programmers prefer the break statement. Unlike C++, Java also offers a labeled break statement that lets you break out of multiple nested loops. Occasionally something weird happens inside a deeply nested loop. In that case, you may want to break completely out of all the nested loops. It is inconvenient to program that simply by adding extra conditions to the various loop tests. From the Library of Hristo Dimov Hristov 3.8 Control Flow 103 Here’s an example that shows the break statement at work. Notice that the label must precede the outermost loop out of which you want to break. It also must be followed by a colon. Scanner in = new Scanner(System.in); int n; read_data: while (. . .) // this loop statement is tagged with the label { . . . for (. . .) // this inner loop is not labeled { System.out.print("Enter a number >= 0: "); n = in.nextInt(); if (n < 0) // should never happen—can't go on break read_data; // break out of read_data loop . . . } } // this statement is executed immediately after the labeled break if (n < 0) // check for bad situation { // deal with bad situation } else { // carry out normal processing } If there is a bad input, the labeled break moves past the end of the labeled block. As with any use of the break statement, you then need to test whether the loop exited normally or as a result of a break. NOTE: Curiously, you can apply a label to any statement, even an if statement or a block statement, like this: label: { . . . if (condition) break label; // exits block . . . } // jumps here when the break statement executes From the Library of Hristo Dimov Hristov 104 Chapter 3 Fundamental Programming Structures in Java Thus, if you are lusting after a goto but you can place a block that ends just before the place to which you want to jump, you can use a break statement! Naturally, we don’t recommend this approach. Note, however, that you can only jump out of a block, never into a block. Finally, there is a continue statement that, like the break statement, breaks the regular flow of control. The continue statement transfers control to the header of the innermost enclosing loop. Here is an example: Scanner in = new Scanner(System.in); while (sum < goal) { System.out.print("Enter a number: "); n = in.nextInt(); if (n < 0) continue; sum += n; // not executed if n < 0 } If n < 0, then the continue statement jumps immediately to the loop header, skipping the remainder of the current iteration. If the continue statement is used in a for loop, it jumps to the “update” part of the for loop. For example: for (count = 1; count <= 100; count++) { System.out.print("Enter a number, -1 to quit: "); n = in.nextInt(); if (n < 0) continue; sum += n; // not executed if n < 0 } If n < 0, then the continue statement jumps to the count++ statement. There is also a labeled form of the continue statement that jumps to the header of the loop with the matching label. TIP: Many programmers find the break and continue statements confusing. These statements are entirely optional—you can always express the same logic without them. In this book, we never use break or continue. From the Library of Hristo Dimov Hristov 3.9 Big Numbers 105 3.9 Big Numbers If the precision of the basic integer and floating-point types is not sufficient, you can turn to a couple of handy classes in the java.math package: BigInteger and BigDecimal. These are classes for manipulating numbers with an arbitrarily long sequence of digits. The BigInteger class implements arbitrary-precision integer arithmetic, and BigDecimal does the same for floating-point numbers. Use the static valueOf method to turn an ordinary number into a big number: BigInteger a = BigInteger.valueOf(100); For longer numbers, use a constructor with a string parameter: BigInteger reallyBig = new BigInteger("222232244629420445529739893461909967206666939096499764990979600"); There are also constants BigInteger.ZERO, BigInteger.ONE, BigInteger.TEN, and, since Java 9, BigInteger.TWO. Unfortunately, you cannot use the familiar mathematical operators such as + and * to combine big numbers. Instead, you must use methods such as add and multiply in the big number classes. BigInteger c = a.add(b); // c = a + b BigInteger d = c.multiply(b.add(BigInteger.valueOf(2))); // d = c * (b + 2) C++ NOTE: Unlike C++, Java has no programmable operator overloading. There was no way for the programmers of the BigInteger class to redefine the + and * operators to give the add and multiply operations of the BigInteger classes. The language designers did overload the + operator to denote concatenation of strings. They chose not to overload other operators, and they did not give Java programmers the opportunity to overload operators in their own classes. Listing 3.6 shows a modification of the lottery odds program of Listing 3.5, updated to work with big numbers. For example, if you are invited to participate in a lottery in which you need to pick 60 numbers out of a possible 490 numbers, you can use this program to tell you your odds of winning. They are 1 in 716395843461995557415116222540092933411717612789263493493351013459481104668848. Good luck! From the Library of Hristo Dimov Hristov 106 Chapter 3 Fundamental Programming Structures in Java The program in Listing 3.5 computed the statement lotteryOdds = lotteryOdds * (n - i + 1) / i; When big numbers are used, the equivalent statement becomes lotteryOdds = lotteryOdds.multiply(BigInteger.valueOf(n - i + 1)).divide(BigInteger.valueOf(i)); Listing 3.6 BigIntegerTest/BigIntegerTest.java 1 2 import java.math.*; import java.util.*; 3 4 5 6 7 8 9 10 11 12 13 /** * This program uses big numbers to compute the odds of winning the grand prize in a lottery. * @version 1.20 2004-02-10 * @author Cay Horstmann */ public class BigIntegerTest { public static void main(String[] args) { Scanner in = new Scanner(System.in); 14 System.out.print("How many numbers do you need to draw? "); int k = in.nextInt(); 15 16 17 System.out.print("What is the highest number you can draw? "); int n = in.nextInt(); 18 19 20 /* * compute binomial coefficient n*(n-1)*(n-2)*...*(n-k+1)/(1*2*3*...*k) */ 21 22 23 24 BigInteger lotteryOdds = BigInteger.valueOf(1); 25 26 for (int i = 1; i <= k; i++) lotteryOdds = lotteryOdds.multiply(BigInteger.valueOf(n - i + 1)).divide( BigInteger.valueOf(i)); 27 28 29 30 System.out.println("Your odds are 1 in " + lotteryOdds + ". Good luck!"); 31 } 32 33 } From the Library of Hristo Dimov Hristov 3.9 Big Numbers 107 java.math.BigInteger 1.1 • • • • • BigInteger BigInteger BigInteger BigInteger BigInteger add(BigInteger other) subtract(BigInteger other) multiply(BigInteger other) divide(BigInteger other) mod(BigInteger other) returns the sum, difference, product, quotient, and remainder of this big integer and other. • BigInteger sqrt() 9 yields the square root of this BigInteger. • int compareTo(BigInteger other) returns 0 if this big integer equals other, a negative result if this big integer is less than other, and a positive result otherwise. • static BigInteger valueOf(long x) returns a big integer whose value equals x. java.math.BigDecimal 1.1 • • • • • BigDecimal BigDecimal BigDecimal BigDecimal BigDecimal add(BigDecimal other) subtract(BigDecimal other) multiply(BigDecimal other) divide(BigDecimal other) divide(BigDecimal other, RoundingMode mode) 5 returns the sum, difference, product, or quotient of this big decimal and other. The first divide method throws an exception if the quotient does not have a finite decimal expansion. To obtain a rounded result, use the second method. The mode RoundingMode.HALF_UP is the rounding mode that you learned in school: round down the digits 0 to 4, round up the digits 5 to 9. It is appropriate for routine calculations. See the API documentation for other rounding modes. • int compareTo(BigDecimal other) returns 0 if this big decimal equals other, a negative result if this big decimal is less than other, and a positive result otherwise. • static BigDecimal valueOf(long x) • static BigDecimal valueOf(long x, int scale) returns a big decimal whose value equals x or x / 10scale. From the Library of Hristo Dimov Hristov 108 Chapter 3 Fundamental Programming Structures in Java 3.10 Arrays Arrays hold sequences of values of the same type. In the following sections, you will see how to work with arrays in Java. 3.10.1 Declaring Arrays An array is a data structure that stores a collection of values of the same type. You access each individual value through an integer index. For example, if a is an array of integers, then a[i] is the ith integer in the array. Declare an array variable by specifying the array type—which is the element type followed by []—and the array variable name. For example, here is the declaration of an array a of integers: int[] a; However, this statement only declares the variable a. It does not yet initialize a with an actual array. Use the new operator to create the array. int[] a = new int[100]; // or var a = new int[100]; This statement declares and initializes an array of 100 integers. The array length need not be a constant: new int[n] creates an array of length n. Once you create an array, you cannot change its length (although you can, of course, change an individual array element). If you frequently need to expand the length of arrays while your program is running, you should use array lists, which are covered in Chapter 5. NOTE: You can define an array variable either as int[] a; or as int a[]; Most Java programmers prefer the former style because it neatly separates the type int[] (integer array) from the variable name. Java has a shortcut for creating an array object and supplying initial values: int[] smallPrimes = { 2, 3, 5, 7, 11, 13 }; Notice that you do not use new with this syntax, and you don’t specify the length. From the Library of Hristo Dimov Hristov 3.10 Arrays 109 A comma after the last value is allowed, which can be convenient for an array to which you keep adding values over time: String[] authors = { "James Gosling", "Bill Joy", "Guy Steele", // add more names here and put a comma after each name }; You can declare an anonymous array: new int[] { 17, 19, 23, 29, 31, 37 } This expression allocates a new array and fills it with the values inside the braces. It counts the number of initial values and sets the array size accordingly. You can use this syntax to reinitialize an array without creating a new variable. For example, smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 }; is shorthand for int[] anonymous = { 17, 19, 23, 29, 31, 37 }; smallPrimes = anonymous; NOTE: It is legal to have arrays of length 0. Such an array can be useful if you write a method that computes an array result and the result happens to be empty. Construct an array of length 0 as new elementType[0] or new elementType[] {} Note that an array of length 0 is not the same as null. 3.10.2 Accessing Array Elements The array elements are numbered from 0 to 99 (and not 1 to 100). Once the array is created, you can fill the elements in an array, for example, by using a loop: int[] a = new int[100]; for (int i = 0; i < 100; i++) a[i] = i; // fills the array with numbers 0 to 99 When you create an array of numbers, all elements are initialized with zero. Arrays of boolean are initialized with false. Arrays of objects are initialized with From the Library of Hristo Dimov Hristov 110 Chapter 3 Fundamental Programming Structures in Java the special value null, which indicates that they do not (yet) hold any objects. This can be surprising for beginners. For example, String[] names = new String[10]; creates an array of ten strings, all of which are null. If you want the array to hold empty strings, you must supply them: for (int i = 0; i < 10; i++) names[i] = ""; CAUTION: If you construct an array with 100 elements and then try to access the element a[100] (or any other index outside the range from 0 to 99), an “array index out of bounds” exception will occur. To find the number of elements of an array, use array.length. For example: for (int i = 0; i < a.length; i++) System.out.println(a[i]); 3.10.3 The “for each” Loop Java has a powerful looping construct that allows you to loop through each element in an array (or any other collection of elements) without having to fuss with index values. The enhanced for loop for (variable : collection) statement sets the given variable to each element of the collection and then executes the statement (which, of course, may be a block). The collection expression must be an array or an object of a class that implements the Iterable interface, such as ArrayList. We discuss array lists in Chapter 5 and the Iterable interface in Chapter 9. For example, for (int element : a) System.out.println(element); prints each element of the array a on a separate line. You should read this loop as “for each element in a”. The designers of the Java language considered using keywords, such as foreach and in. But this loop was a late addition to the Java language, and in the end nobody wanted to break the old code that already contained methods or variables with these names (such as System.in). From the Library of Hristo Dimov Hristov 3.10 Arrays 111 Of course, you could achieve the same effect with a traditional for loop: for (int i = 0; i < a.length; i++) System.out.println(a[i]); However, the “for each” loop is more concise and less error-prone, as you don’t have to worry about those pesky start and end index values. NOTE: The loop variable of the “for each” loop traverses the elements of the array, not the index values. The “for each” loop is a pleasant improvement over the traditional loop if you need to process all elements in a collection. However, there are still plenty of opportunities to use the traditional for loop. For example, you might not want to traverse the entire collection, or you may need the index value inside the loop. TIP: There is an even easier way to print all values of an array, using the toString method of the Arrays class. The call Arrays.toString(a) returns a string containing the array elements, enclosed in brackets and separated by commas, such as "[2, 3, 5, 7, 11, 13]". To print the array, simply call System.out.println(Arrays.toString(a)); 3.10.4 Array Copying You can copy one array variable into another, but then both variables refer to the same array: int[] luckyNumbers = smallPrimes; luckyNumbers[5] = 12; // now smallPrimes[5] is also 12 Figure 3.14 shows the result. If you actually want to copy all values of one array into a new array, use the copyOf method in the Arrays class: int[] copiedLuckyNumbers = Arrays.copyOf(luckyNumbers, luckyNumbers.length); The second parameter is the length of the new array. A common use of this method is to increase the size of an array: luckyNumbers = Arrays.copyOf(luckyNumbers, 2 * luckyNumbers.length); The additional elements are filled with 0 if the array contains numbers, false if the array contains boolean values. Conversely, if the length is less than the length of the original array, only the initial values are copied. From the Library of Hristo Dimov Hristov 112 Chapter 3 Fundamental Programming Structures in Java Figure 3.14 Copying an array variable C++ NOTE: A Java array is quite different from a C++ array on the stack. It is, however, essentially the same as a pointer to an array allocated on the heap. That is, int[] a = new int[100]; // Java is not the same as int a[100]; // C++ but rather int* a = new int[100]; // C++ In Java, the [] operator is predefined to perform bounds checking. Furthermore, there is no pointer arithmetic—you can’t increment a to point to the next element in the array. 3.10.5 Command-Line Parameters You have already seen one example of a Java array repeated quite a few times. Every Java program has a main method with a String[] args parameter. This parameter indicates that the main method receives an array of strings— namely, the arguments specified on the command line. For example, consider this program: public class Message { public static void main(String[] args) { if (args.length == 0 || args[0].equals("-h")) System.out.print("Hello,"); From the Library of Hristo Dimov Hristov 3.10 Arrays 113 else if (args[0].equals("-g")) System.out.print("Goodbye,"); // print the other command-line arguments for (int i = 1; i < args.length; i++) System.out.print(" " + args[i]); System.out.println("!"); } } If the program is called as java Message -g cruel world then the args array has the following contents: args[0]: "-g" args[1]: "cruel" args[2]: "world" The program prints the message Goodbye, cruel world! C++ NOTE: In the main method of a Java program, the name of the program is not stored in the args array. For example, when you start up a program as java Message -h world from the command line, then args[0] will be "-h" and not "Message" or "java". 3.10.6 Array Sorting To sort an array of numbers, you can use one of the sort methods in the Arrays class: int[] a = new int[10000]; . . . Arrays.sort(a) This method uses a tuned version of the QuickSort algorithm that is claimed to be very efficient on most data sets. The Arrays class provides several other convenience methods for arrays that are included in the API notes at the end of this section. The program in Listing 3.7 puts arrays to work. This program draws a random combination of numbers for a lottery game. For example, if you play a “choose 6 numbers from 49” lottery, the program might print this: From the Library of Hristo Dimov Hristov 114 Chapter 3 Fundamental Programming Structures in Java Bet the following combination. It'll make you rich! 4 7 8 19 30 44 To select such a random set of numbers, we first fill an array numbers with the values 1, 2, . . ., n: int[] numbers = new int[n]; for (int i = 0; i < numbers.length; i++) numbers[i] = i + 1; A second array holds the numbers to be drawn: int[] result = new int[k]; Now we draw k numbers. The Math.random method returns a random floatingpoint number that is between 0 (inclusive) and 1 (exclusive). By multiplying the result with n, we obtain a random number between 0 and n – 1. int r = (int) (Math.random() * n); We set the ith result to be the number at that index. Initially, that is just r + 1, but as you’ll see presently, the contents of the numbers array are changed after each draw. result[i] = numbers[r]; Now we must be sure never to draw that number again—all lottery numbers must be distinct. Therefore, we overwrite numbers[r] with the last number in the array and reduce n by 1. numbers[r] = numbers[n - 1]; n--; The point is that in each draw we pick an index, not the actual value. The index points into an array that contains the values that have not yet been drawn. After drawing k lottery numbers, we sort the result array for a more pleasing output: Arrays.sort(result); for (int r : result) System.out.println(r); From the Library of Hristo Dimov Hristov 3.10 Arrays 115 Listing 3.7 LotteryDrawing/LotteryDrawing.java 1 import java.util.*; 2 3 4 5 6 7 8 9 10 11 12 /** * This program demonstrates array manipulation. * @version 1.20 2004-02-10 * @author Cay Horstmann */ public class LotteryDrawing { public static void main(String[] args) { Scanner in = new Scanner(System.in); 13 System.out.print("How many numbers do you need to draw? "); int k = in.nextInt(); 14 15 16 System.out.print("What is the highest number you can draw? "); int n = in.nextInt(); 17 18 19 // fill an array with numbers 1 2 3 . . . n int[] numbers = new int[n]; for (int i = 0; i < numbers.length; i++) numbers[i] = i + 1; 20 21 22 23 24 // draw k numbers and put them into a second array int[] result = new int[k]; for (int i = 0; i < result.length; i++) { // make a random index between 0 and n - 1 int r = (int) (Math.random() * n); 25 26 27 28 29 30 31 // pick the element at the random location result[i] = numbers[r]; 32 33 34 // move the last element into the random location numbers[r] = numbers[n - 1]; n--; 35 36 37 } 38 39 // print the sorted array Arrays.sort(result); System.out.println("Bet the following combination. It'll make you rich!"); for (int r : result) System.out.println(r); 40 41 42 43 44 } 45 46 } From the Library of Hristo Dimov Hristov 116 Chapter 3 Fundamental Programming Structures in Java java.util.Arrays 1.2 • static String toString(xxx[] a) 5 returns a string with the elements of a, enclosed in brackets and delimited by commas. In this and the following methods, the component type xxx of the array can be int, long, short, char, byte, boolean, float, or double. • static xxx[] copyOf(xxx[] a, int end) 6 • static xxx[] copyOfRange(xxx[] a, int start, int end) 6 returns an array of the same type as a, of length either end or end – start, filled with the values of a. If end is larger than a.length, the result is padded with 0 or false values. • static void sort(xxx[] a) sorts the array, using a tuned QuickSort algorithm. • static int binarySearch(xxx[] a, xxx v) • static int binarySearch(xxx[] a, int start, int end, xxx v) 6 uses the binary search algorithm to search for the value v in the sorted array a. If v is found, its index is returned. Otherwise, a negative value r is returned; –r – 1 is the spot at which v should be inserted to keep a sorted. • static void fill(xxx[] a, xxx v) sets all elements of the array to v. • static boolean equals(xxx[] a, xxx[] b) returns true if the arrays have the same length and if the elements at corresponding indexes match. 3.10.7 Multidimensional Arrays Multidimensional arrays use more than one index to access array elements. They are used for tables and other more complex arrangements. You can safely skip this section until you have a need for this storage mechanism. Suppose you want to make a table of numbers that shows how much an investment of $10,000 will grow under different interest rate scenarios in which interest is paid annually and reinvested (Table 3.8). You can store this information in a two-dimensional array (matrix), which we call balances. Declaring a two-dimensional array in Java is simple enough. For example: double[][] balances; From the Library of Hristo Dimov Hristov 3.10 Arrays 117 Table 3.8 Growth of an Investment at Different Interest Rates 10% 11% 12% 13% 14% 15% 10,000.00 10,000.00 10,000.00 10,000.00 10,000.00 10,000.00 11,000.00 11,100.00 11,200.00 11,300.00 11,400.00 11,500.00 12,100.00 12,321.00 12,544.00 12,769.00 12,996.00 13,225.00 13,310.00 13,676.31 14,049.28 14,428.97 14,815.44 15,208.75 14,641.00 15,180.70 15,735.19 16,304.74 16,889.60 17,490.06 16,105.10 16,850.58 17,623.42 18,424.35 19,254.15 20,113.57 17,715.61 18,704.15 19,738.23 20,819.52 21,949.73 23,130.61 19,487.17 20,761.60 22,106.81 23,526.05 25,022.69 26,600.20 21,435.89 23,045.38 24,759.63 26,584.44 28,525.86 30,590.23 23,579.48 25,580.37 27,730.79 30,040.42 32,519.49 35,178.76 You cannot use the array until you initialize it. In this case, you can do the initialization as follows: balances = new double[NYEARS][NRATES]; In other cases, if you know the array elements, you can use a shorthand notation for initializing a multidimensional array without a call to new. For example: int[][] magicSquare = { {16, 3, 2, 13}, {5, 10, 11, 8}, {9, 6, 7, 12}, {4, 15, 14, 1} }; Once the array is initialized, you can access individual elements by supplying two pairs of brackets—for example, balances[i][j]. The example program stores a one-dimensional array interest of interest rates and a two-dimensional array balances of account balances, one for each year and interest rate. We initialize the first row of the array with the initial balance: for (int j = 0; j < balances[0].length; j++) balances[0][j] = 10000; From the Library of Hristo Dimov Hristov 118 Chapter 3 Fundamental Programming Structures in Java Then we compute the other rows, as follows: for (int i = 1; i < balances.length; i++) { for (int j = 0; j < balances[i].length; j++) { double oldBalance = balances[i - 1][j]; double interest = . . .; balances[i][j] = oldBalance + interest; } } Listing 3.8 shows the full program. NOTE: A “for each” loop does not automatically loop through all elements in a two-dimensional array. Instead, it loops through the rows, which are themselves one-dimensional arrays. To visit all elements of a two-dimensional array a, nest two loops, like this: for (double[] row : a) for (double value : row) do something with value TIP: To print out a quick-and-dirty list of the elements of a two-dimensional array, call System.out.println(Arrays.deepToString(a)); The output is formatted like this: [[16, 3, 2, 13], [5, 10, 11, 8], [9, 6, 7, 12], [4, 15, 14, 1]] Listing 3.8 CompoundInterest/CompoundInterest.java 1 2 3 4 5 6 7 8 9 /** * This program shows how to store tabular data in a 2D array. * @version 1.40 2004-02-10 * @author Cay Horstmann */ public class CompoundInterest { public static void main(String[] args) { From the Library of Hristo Dimov Hristov 3.10 Arrays 119 final double STARTRATE = 10; final int NRATES = 6; final int NYEARS = 10; 10 11 12 13 // set interest rates to 10 . . . 15% double[] interestRate = new double[NRATES]; for (int j = 0; j < interestRate.length; j++) interestRate[j] = (STARTRATE + j) / 100.0; 14 15 16 17 18 double[][] balances = new double[NYEARS][NRATES]; 19 20 // set initial balances to 10000 for (int j = 0; j < balances[0].length; j++) balances[0][j] = 10000; 21 22 23 24 // compute interest for future years for (int i = 1; i < balances.length; i++) { for (int j = 0; j < balances[i].length; j++) { // get last year's balances from previous row double oldBalance = balances[i - 1][j]; 25 26 27 28 29 30 31 32 // compute interest double interest = oldBalance * interestRate[j]; 33 34 35 // compute this year's balances balances[i][j] = oldBalance + interest; 36 37 } 38 } 39 40 // print one row of interest rates for (int j = 0; j < interestRate.length; j++) System.out.printf("%9.0f%%", 100 * interestRate[j]); 41 42 43 44 System.out.println(); 45 46 // print balance table for (double[] row : balances) { // print table row for (double b : row) System.out.printf("%10.2f", b); 47 48 49 50 51 52 53 System.out.println(); 54 } 55 } 56 57 } From the Library of Hristo Dimov Hristov 120 Chapter 3 Fundamental Programming Structures in Java 3.10.8 Ragged Arrays So far, what you have seen is not too different from other programming languages. But there is actually something subtle going on behind the scenes that you can sometimes turn to your advantage: Java has no multidimensional arrays at all, only one-dimensional arrays. Multidimensional arrays are faked as “arrays of arrays.” For example, the balances array in the preceding example is actually an array that contains ten elements, each of which is an array of six floating-point numbers (Figure 3.15). Figure 3.15 A two-dimensional array From the Library of Hristo Dimov Hristov 3.10 Arrays 121 The expression balances[i] refers to the ith subarray—that is, the ith row of the table. It is itself an array, and balances[i][j] refers to the jth element of that array. Since rows of arrays are individually accessible, you can actually swap them! double[] temp = balances[i]; balances[i] = balances[i + 1]; balances[i + 1] = temp; It is also easy to make “ragged” arrays—that is, arrays in which different rows have different lengths. Here is the standard example. Let us make an array in which the element at row i and column j equals the number of possible outcomes of a “choose j numbers from i numbers” lottery. 1 1 1 1 1 1 1 1 2 3 4 5 6 1 3 6 10 15 1 4 1 10 5 1 20 15 6 1 As j can never be larger than i, the matrix is triangular. The ith row has i + 1 elements. (We allow choosing 0 elements; there is one way to make such a choice.) To build this ragged array, first allocate the array holding the rows: int[][] odds = new int[NMAX + 1][]; Next, allocate the rows: for (int n = 0; n <= NMAX; n++) odds[n] = new int[n + 1]; Now that the array is allocated, we can access the elements in the normal way, provided we do not overstep the bounds: for (int n = 0; n < odds.length; n++) for (int k = 0; k < odds[n].length; k++) { // compute lotteryOdds . . . odds[n][k] = lotteryOdds; } Listing 3.9 gives the complete program. From the Library of Hristo Dimov Hristov 122 Chapter 3 Fundamental Programming Structures in Java C++ NOTE: In C++, the Java declaration double[][] balances = new double[10][6]; // Java is not the same as double balances[10][6]; // C++ or even double (*balances)[6] = new double[10][6]; // C++ Instead, an array of ten pointers is allocated: double** balances = new double*[10]; // C++ Then, each element in the pointer array is filled with an array of six numbers: for (i = 0; i < 10; i++) balances[i] = new double[6]; Mercifully, this loop is automatic when you ask for a new double[10][6]. When you want ragged arrays, you allocate the row arrays separately. Listing 3.9 LotteryArray/LotteryArray.java 1 2 3 4 5 6 7 8 9 10 /** * This program demonstrates a triangular array. * @version 1.20 2004-02-10 * @author Cay Horstmann */ public class LotteryArray { public static void main(String[] args) { final int NMAX = 10; 11 12 13 14 15 // allocate triangular array int[][] odds = new int[NMAX + 1][]; for (int n = 0; n <= NMAX; n++) odds[n] = new int[n + 1]; 16 17 18 19 20 21 22 23 // fill triangular array for (int n = 0; n < odds.length; n++) for (int k = 0; k < odds[n].length; k++) { /* * compute binomial coefficient n*(n-1)*(n-2)*...*(n-k+1)/(1*2*3*...*k) */ From the Library of Hristo Dimov Hristov 3.10 Arrays 123 int lotteryOdds = 1; for (int i = 1; i <= k; i++) lotteryOdds = lotteryOdds * (n - i + 1) / i; 24 25 26 27 odds[n][k] = lotteryOdds; 28 } 29 30 // print triangular array for (int[] row : odds) { for (int odd : row) System.out.printf("%4d", odd); System.out.println(); } 31 32 33 34 35 36 37 } 38 39 } You have now seen the fundamental programming structures of the Java language. The next chapter covers object-oriented programming in Java. From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov CHAPTER 4 Objects and Classes In this chapter • 4.1 Introduction to Object-Oriented Programming, page 126 • 4.2 Using Predefined Classes, page 131 • 4.3 Defining Your Own Classes, page 141 • 4.4 Static Fields and Methods, page 156 • 4.5 Method Parameters, page 163 • 4.6 Object Construction, page 170 • 4.7 Packages, page 180 • 4.8 JAR Files, page 192 • 4.9 Documentation Comments, page 198 • 4.10 Class Design Hints, page 204 In this chapter, we • Introduce you to object-oriented programming; • Show you how you can create objects that belong to classes from the standard Java library; and • Show you how to write your own classes. If you do not have a background in object-oriented programming, you will want to read this chapter carefully. Object-oriented programming requires a different way of thinking than procedural languages. The transition is not 125 From the Library of Hristo Dimov Hristov 126 Chapter 4 Objects and Classes always easy, but you do need some familiarity with object concepts to go further with Java. For experienced C++ programmers, this chapter, like the previous chapter, presents familiar information; however, there are enough differences between the two languages that you should read the later sections of this chapter carefully. You’ll find the C++ notes helpful for making the transition. 4.1 Introduction to Object-Oriented Programming Object-oriented programming, or OOP for short, is the dominant programming paradigm these days, having replaced the “structured” or procedural programming techniques that were developed in the 1970s. Since Java is objectoriented, you have to be familiar with OOP to become productive with Java. An object-oriented program is made of objects. Each object has a specific functionality, exposed to its users, and a hidden implementation. Many objects in your programs will be taken “off-the-shelf” from a library; others will be custom-designed. Whether you build an object or buy it might depend on your budget or time. But, basically, as long as an object satisfies your specifications, you don’t care how the functionality is implemented. Traditional structured programming consists of designing a set of procedures (or algorithms) to solve a problem. Once the procedures are determined, the traditional next step was to find appropriate ways to store the data. This is why the designer of the Pascal language, Niklaus Wirth, called his famous book on programming Algorithms + Data Structures = Programs (Prentice Hall, 1975). Notice that in Wirth’s title, algorithms come first, and data structures second. This reflects the way programmers worked at that time. First, they decided on the procedures for manipulating the data; then, they decided what structure to impose on the data to make the manipulations easier. OOP reverses the order: puts the data first, then looks at the algorithms to operate on the data. For small problems, the breakdown into procedures works very well. But objects are more appropriate for larger problems. Consider a simple web browser. It might require 2,000 procedures for its implementation, all of which manipulate a set of global data. In the object-oriented style, there might be 100 classes with an average of 20 methods per class (see Figure 4.1). This structure is much easier for a programmer to grasp. It is also much easier to find bugs in. Suppose the data of a particular object is in an incorrect state. It is far easier to search for the culprit among the 20 methods that had access to that data item than among 2,000 procedures. From the Library of Hristo Dimov Hristov 4.1 Introduction to Object-Oriented Programming 127 Figure 4.1 Procedural vs. OO programming 4.1.1 Classes A class specifies how objects are made. Think of classes as cookie cutters; objects are the cookies themselves. When you construct an object from a class, you are said to have created an instance of the class. As you have seen, all code that you write in Java is inside a class. The standard Java library supplies several thousand classes for such diverse purposes as user interface design, dates and calendars, and network programming. Nonetheless, in Java you still have to create your own classes to describe the objects of your application’s problem domain. Encapsulation (sometimes called information hiding) is a key concept in working with objects. Formally, encapsulation is simply combining data and behavior in one package and hiding the implementation details from the users of the object. The bits of data in an object are called its instance fields, and the procedures that operate on the data are called its methods. A specific object that is an instance of a class will have specific values of its instance fields. The set of those values is the current state of the object. Whenever you invoke a method on an object, its state may change. The key to making encapsulation work is to have methods never directly access instance fields in a class other than their own. Programs should interact with object data only through the object’s methods. Encapsulation is the way to From the Library of Hristo Dimov Hristov 128 Chapter 4 Objects and Classes give an object its “black box” behavior, which is the key to reuse and reliability. This means a class may totally change how it stores its data, but as long as it continues to use the same methods to manipulate the data, no other object will know or care. When you start writing your own classes in Java, another tenet of OOP will make this easier: Classes can be built by extending other classes. Java, in fact, comes with a “cosmic superclass” called Object. All other classes extend this class. You will learn more about the Object class in the next chapter. When you extend an existing class, the new class has all the properties and methods of the class that you extend. You then supply new methods and data fields that apply to your new class only. The concept of extending a class to obtain another class is called inheritance. See the next chapter for more on inheritance. 4.1.2 Objects To work with OOP, you should be able to identify three key characteristics of objects: • The object’s behavior—what can you do with this object, or what methods can you apply to it? • The object’s state—how does the object react when you invoke those methods? • The object’s identity—how is the object distinguished from others that may have the same behavior and state? All objects that are instances of the same class share a family resemblance by supporting the same behavior. The behavior of an object is defined by the methods that you can call. Next, each object stores information about what it currently looks like. This is the object’s state. An object’s state may change over time, but not spontaneously. A change in the state of an object must be a consequence of method calls. (If an object’s state changed without a method call on that object, someone broke encapsulation.) However, the state of an object does not completely describe it, because each object has a distinct identity. For example, in an order processing system, two orders are distinct even if they request identical items. Notice that the individual objects that are instances of a class always differ in their identity and usually differ in their state. These key characteristics can influence each other. For example, the state of an object can influence its behavior. (If an order is “shipped” or “paid,” it may From the Library of Hristo Dimov Hristov 4.1 Introduction to Object-Oriented Programming 129 reject a method call that asks it to add or remove items. Conversely, if an order is “empty”—that is, no items have yet been ordered—it should not allow itself to be shipped.) 4.1.3 Identifying Classes In a traditional procedural program, you start the process at the top, with the main function. When designing an object-oriented system, there is no “top,” and newcomers to OOP often wonder where to begin. The answer is: Identify your classes and then add methods to each class. A simple rule of thumb in identifying classes is to look for nouns in the problem analysis. Methods, on the other hand, correspond to verbs. For example, in an order-processing system, some of the nouns are • • • • • Item Order Shipping address Payment Account These nouns may lead to the classes Item, Order, and so on. Next, look for verbs. Items are added to orders. Orders are shipped or canceled. Payments are applied to orders. With each verb, such as “add,” “ship,” “cancel,” or “apply,” you identify the object that has the major responsibility for carrying it out. For example, when a new item is added to an order, the order object should be the one in charge because it knows how it stores and sorts items. That is, add should be a method of the Order class that takes an Item object as a parameter. Of course, the “noun and verb” is but a rule of thumb; only experience can help you decide which nouns and verbs are the important ones when building your classes. 4.1.4 Relationships between Classes The most common relationships between classes are • Dependence (“uses–a”) • Aggregation (“has–a”) • Inheritance (“is–a”) From the Library of Hristo Dimov Hristov 130 Chapter 4 Objects and Classes The dependence, or “uses–a” relationship, is the most obvious and also the most general. For example, the Order class uses the Account class because Order objects need to access Account objects to check for credit status. But the Item class does not depend on the Account class, because Item objects never need to worry about customer accounts. Thus, a class depends on another class if its methods use or manipulate objects of that class. Try to minimize the number of classes that depend on each other. The point is, if a class A is unaware of the existence of a class B, it is also unconcerned about any changes to B. (And this means that changes to B do not introduce bugs into A.) In software engineering terminology, you want to minimize the coupling between classes. The aggregation, or “has–a” relationship, is easy to understand because it is concrete; for example, an Order object contains Item objects. Containment means that objects of class A contain objects of class B. NOTE: Some methodologists view the concept of aggregation with disdain and prefer to use a more general “association” relationship. From the point of view of modeling, that is understandable. But for programmers, the “has–a” relationship makes a lot of sense. We like to use aggregation for another reason as well: The standard notation for associations is less clear. See Table 4.1. The inheritance, or “is–a” relationship, expresses a relationship between a more special and a more general class. For example, a RushOrder class inherits from an Order class. The specialized RushOrder class has special methods for priority handling and a different method for computing shipping charges, but its other methods, such as adding items and billing, are inherited from the Order class. In general, if class A extends class B, class A inherits methods from class B but has more capabilities. (See the next chapter in which we discuss this important notion at some length.) Many programmers use the UML (Unified Modeling Language) notation to draw class diagrams that describe the relationships between classes. You can see an example of such a diagram in Figure 4.2. You draw classes as rectangles, and relationships as arrows with various adornments. Table 4.1 shows the most common UML arrow styles. From the Library of Hristo Dimov Hristov 4.2 Using Predefined Classes 131 Figure 4.2 A class diagram Table 4.1 UML Notation for Class Relationships Relationship UML Connector Inheritance Interface implementation Dependency Aggregation Association Directed association 4.2 Using Predefined Classes You can’t do anything in Java without classes, and you have already seen several classes at work. However, not all of these show off the typical features From the Library of Hristo Dimov Hristov 132 Chapter 4 Objects and Classes of object orientation. Take, for example, the Math class. You have seen that you can use methods of the Math class, such as Math.random, without needing to know how they are implemented—all you need to know is the name and parameters (if any). That’s the point of encapsulation, and it will certainly be true of all classes. But the Math class only encapsulates functionality; it neither needs nor hides data. Since there is no data, you do not need to worry about making objects and initializing their instance fields—there aren’t any! In the next section, we will look at a more typical class, the Date class. You will see how to construct objects and call methods of this class. 4.2.1 Objects and Object Variables To work with objects, you first construct them and specify their initial state. Then you apply methods to the objects. In the Java programming language, you use constructors to construct new instances. A constructor is a special method whose purpose is to construct and initialize objects. Let us look at an example. The standard Java library contains a Date class. Its objects describe points in time, such as December 31, 1999, 23:59:59 GMT. NOTE: You may be wondering: Why use a class to represent dates rather than (as in some languages) a built-in type? For example, Visual Basic has a built-in date type, and programmers can specify dates in the format #6/1/1995#. On the surface, this sounds convenient—programmers can simply use the built-in date type without worrying about classes. But actually, how suitable is the Visual Basic design? In some locales, dates are specified as month/day/year, in others as day/month/year. Are the language designers really equipped to foresee these kinds of issues? If they do a poor job, the language becomes an unpleasant muddle, but unhappy programmers are powerless to do anything about it. With classes, the design task is offloaded to a library designer. If the class is not perfect, other programmers can easily write their own classes to enhance or replace the system classes. (To prove the point: The Java date library started out a bit muddled, and it has been redesigned twice.) Constructors always have the same name as the class name. Thus, the constructor for the Date class is called Date. To construct a Date object, combine the constructor with the new operator, as follows: new Date() This expression constructs a new object. The object is initialized to the current date and time. From the Library of Hristo Dimov Hristov 4.2 Using Predefined Classes 133 If you like, you can pass the object to a method: System.out.println(new Date()); Alternatively, you can apply a method to the object that you just constructed. One of the methods of the Date class is the toString method. That method yields a string representation of the date. Here is how you would apply the toString method to a newly constructed Date object: String s = new Date().toString(); In these two examples, the constructed object is used only once. Usually, you will want to hang on to the objects that you construct so that you can keep using them. Simply store the object in a variable: Date birthday = new Date(); Figure 4.3 shows the object variable birthday that refers to the newly constructed object. Figure 4.3 Creating a new object There is an important difference between objects and object variables. For example, the statement Date deadline; // deadline doesn't refer to any object defines an object variable, deadline, that can refer to objects of type Date. It is important to realize that the variable deadline is not an object and, in fact, does not even refer to an object yet. You cannot use any Date methods on this variable at this time. The statement s = deadline.toString(); // not yet would cause a compile-time error. From the Library of Hristo Dimov Hristov 134 Chapter 4 Objects and Classes You must first initialize the deadline variable. You have two choices. Of course, you can initialize the variable so that it refers to a newly constructed object: deadline = new Date(); Or you can set the variable to refer to an existing object: deadline = birthday; Now both variables refer to the same object (see Figure 4.4). Figure 4.4 Object variables that refer to the same object It is important to realize that an object variable doesn’t actually contain an object. It only refers to an object. In Java, the value of any object variable is a reference to an object that is stored elsewhere. The return value of the new operator is also a reference. A statement such as Date deadline = new Date(); has two parts. The expression new Date() makes an object of type Date, and its value is a reference to that newly created object. That reference is then stored in the deadline variable. You can explicitly set an object variable to null to indicate that it currently refers to no object. deadline = null; . . . if (deadline != null) System.out.println(deadline); From the Library of Hristo Dimov Hristov 4.2 Using Predefined Classes 135 We will discuss null in more detail in Section 4.3.6, “Working with null References,” on p. 148. C++ NOTE: Some people mistakenly believe that Java object variables behave like C++ references. But in C++ there are no null references, and references cannot be assigned. You should think of Java object variables as analogous to object pointers in C++. For example, Date birthday; // Java is really the same as Date* birthday; // C++ Once you make this association, everything falls into place. Of course, a Date* pointer isn’t initialized until you initialize it with a call to new. The syntax is almost the same in C++ and Java. Date* birthday = new Date(); // C++ If you copy one variable to another, then both variables refer to the same date—they are pointers to the same object. The equivalent of the Java null reference is the C++ NULL pointer. All Java objects live on the heap. When an object contains another object variable, it contains just a pointer to yet another heap object. In C++, pointers make you nervous because they are so error-prone. It is easy to create bad pointers or to mess up memory management. In Java, these problems simply go away. If you use an uninitialized pointer, the runtime system will reliably generate a runtime error instead of producing random results. You don’t have to worry about memory management, because the garbage collector takes care of it. C++ makes quite an effort, with its support for copy constructors and assignment operators, to allow the implementation of objects that copy themselves automatically. For example, a copy of a linked list is a new linked list with the same contents but with an independent set of links. This makes it possible to design classes with the same copy behavior as the built-in types. In Java, you must use the clone method to get a complete copy of an object. 4.2.2 The LocalDate Class of the Java Library In the preceding examples, we used the Date class that is a part of the standard Java library. An instance of the Date class has a state—namely, a particular point in time. From the Library of Hristo Dimov Hristov 136 Chapter 4 Objects and Classes Although you don’t need to know this when you use the Date class, the time is represented by the number of milliseconds (positive or negative) from a fixed point, the so-called epoch, which is 00:00:00 UTC, January 1, 1970. UTC is the Coordinated Universal Time, the scientific time standard which is, for practical purposes, the same as the more familiar GMT, or Greenwich Mean Time. But as it turns out, the Date class is not very useful for manipulating the kind of calendar information that humans use for dates, such as “December 31, 1999”. This particular description of a day follows the Gregorian calendar, which is the calendar used in most countries of the world. The same point in time would be described quite differently in the Chinese or Hebrew lunar calendars, not to mention the calendar used by your customers from Mars. NOTE: Throughout human history, civilizations grappled with the design of calendars to attach names to dates and bring order to the solar and lunar cycles. For a fascinating explanation of calendars around the world, from the French Revolutionary calendar to the Mayan long count, see Calendrical Calculations by Nachum Dershowitz and Edward M. Reingold (Cambridge University Press, 3rd ed., 2007). The library designers decided to separate the concerns of keeping time and attaching names to points in time. Therefore, the standard Java library contains two separate classes: the Date class, which represents a point in time, and the LocalDate class, which expresses days in the familiar calendar notation. Java 8 introduced quite a few other classes for manipulating various aspects of date and time—see Chapter 6 of Volume II. Separating time measurement from calendars is good object-oriented design. In general, it is a good idea to use different classes to express different concepts. You do not use a constructor to construct objects of the LocalDate class. Instead, use static factory methods that call constructors on your behalf. The expression LocalDate.now() constructs a new object that represents the date at which the object was constructed. You can construct an object for a specific date by supplying year, month, and day: LocalDate.of(1999, 12, 31) From the Library of Hristo Dimov Hristov 4.2 Using Predefined Classes 137 Of course, you will usually want to store the constructed object in an object variable: LocalDate newYearsEve = LocalDate.of(1999, 12, 31); Once you have a LocalDate object, you can find out the year, month, and day with the methods getYear, getMonthValue, and getDayOfMonth: int year = newYearsEve.getYear(); // 1999 int month = newYearsEve.getMonthValue(); // 12 int day = newYearsEve.getDayOfMonth(); // 31 This may seem pointless because they are the very same values that you just used to construct the object. But sometimes, you have a date that has been computed, and then you will want to invoke those methods to find out more about it. For example, the plusDays method yields a new LocalDate that is a given number of days away from the object to which you apply it: LocalDate aThousandDaysLater = newYearsEve.plusDays(1000); year = aThousandDaysLater.getYear(); // 2002 month = aThousandDaysLater.getMonthValue(); // 09 day = aThousandDaysLater.getDayOfMonth(); // 26 The LocalDate class has encapsulated instance fields to maintain the date to which it is set. Without looking at the source code, it is impossible to know the representation that the class uses internally. But, of course, the point of encapsulation is that this doesn’t matter. What matters are the methods that a class exposes. NOTE: Actually, the Date class also has methods to get the day, month, and year, called getDay, getMonth, and getYear, but these methods are deprecated. A method is deprecated when a library designer realizes that the method should have never been introduced in the first place. These methods were a part of the Date class before the library designers realized that it makes more sense to supply separate classes to deal with calendars. When an earlier set of calendar classes was introduced in Java 1.1, the Date methods were tagged as deprecated. You can still use them in your programs, but you will get unsightly compiler warnings if you do. It is a good idea to stay away from using deprecated methods because they may be removed in a future version of the library. TIP: The JDK provides the jdeprscan tool for checking whether your code uses deprecated features of the Java API. See https://docs.oracle.com/javase /9/tools/jdeprscan.htm for instructions. From the Library of Hristo Dimov Hristov 138 Chapter 4 Objects and Classes 4.2.3 Mutator and Accessor Methods Have another look at the plusDays method call that you saw in the preceding section: LocalDate aThousandDaysLater = newYearsEve.plusDays(1000); What happens to newYearsEve after the call? Has it been changed to be a thousand days later? As it turns out, it has not. The plusDays method yields a new LocalDate object, which is then assigned to the aThousandDaysLater variable. The original object remains unchanged. We say that the plusDays method does not mutate the object on which it is invoked. (This is similar to the toUpperCase method of the String class that you saw in Chapter 3. When you call toUpperCase on a string, that string stays the same, and a new string with uppercase characters is returned.) An earlier version of the Java library had a different class for dealing with calendars, called GregorianCalendar. Here is how you add a thousand days to a date represented by that class: GregorianCalendar someDay = new GregorianCalendar(1999, 11, 31); // odd feature of that class: month numbers go from 0 to 11 someDay.add(Calendar.DAY_OF_MONTH, 1000); Unlike the LocalDate.plusDays method, the GregorianCalendar.add method is a mutator method. After invoking it, the state of the someDay object has changed. Here is how you can find out the new state: year = someDay.get(Calendar.YEAR); // 2002 month = someDay.get(Calendar.MONTH) + 1; // 09 day = someDay.get(Calendar.DAY_OF_MONTH); // 26 That’s why we called the variable someDay and not newYearsEve—it no longer is new year’s eve after calling the mutator method. In contrast, methods that only access objects without modifying them are sometimes called accessor methods. For example, LocalDate.getYear and GregorianCalendar.get are accessor methods. C++ NOTE: In C++, the const suffix denotes accessor methods. A method that is not declared as const is assumed to be a mutator. However, in the Java programming language, no special syntax distinguishes accessors from mutators. We finish this section with a program that puts the LocalDate class to work. The program displays a calendar for the current month, like this: From the Library of Hristo Dimov Hristov 4.2 Using Predefined Classes 139 Mon Tue Wed Thu Fri Sat Sun 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26* 27 28 29 30 The current day is marked with an asterisk (*). As you can see, the program needs to know how to compute the length of a month and the weekday of a given day. Let us go through the key steps of the program. First, we construct an object that is initialized with the current date. LocalDate date = LocalDate.now(); We capture the current month and day. int month = date.getMonthValue(); int today = date.getDayOfMonth(); Then we set date to the first of the month and get the weekday of that date. date = date.minusDays(today - 1); // set to start of month DayOfWeek weekday = date.getDayOfWeek(); int value = weekday.getValue(); // 1 = Monday, . . . , 7 = Sunday The variable weekday is set to an object of type DayOfWeek. We call the getValue method of that object to get a numerical value for the weekday. This yields an integer that follows the international convention where the weekend comes at the end of the week, returning 1 for Monday, 2 for Tuesday, and so on. Sunday has value 7. Note that the first line of the calendar is indented, so that the first day of the month falls on the appropriate weekday. Here is the code to print the header and the indentation for the first line: System.out.println("Mon Tue Wed Thu Fri Sat Sun"); for (int i = 1; i < value; i++) System.out.print(" "); Now, we are ready to print the body of the calendar. We enter a loop in which date traverses the days of the month. In each iteration, we print the date value. If date is today, the date is marked with an *. Then, we advance date to the next day. When we reach the beginning of each new week, we print a new line: From the Library of Hristo Dimov Hristov 140 Chapter 4 Objects and Classes while (date.getMonthValue() == month) { System.out.printf("%3d", date.getDayOfMonth()); if (date.getDayOfMonth() == today) System.out.print("*"); else System.out.print(" "); date = date.plusDays(1); if (date.getDayOfWeek().getValue() == 1) System.out.println(); } When do we stop? We don’t know whether the month has 31, 30, 29, or 28 days. Instead, we keep iterating while date is still in the current month. Listing 4.1 shows the complete program. As you can see, the LocalDate class makes it possible to write a calendar program that takes care of complexities such as weekdays and the varying month lengths. You don’t need to know how the LocalDate class computes months and weekdays. You just use the interface of the class—the methods such as plusDays and getDayOfWeek. The point of this example program is to show you how you can use the interface of a class to carry out fairly sophisticated tasks without having to know the implementation details. Listing 4.1 CalendarTest/CalendarTest.java 1 import java.time.*; 2 3 4 5 6 7 8 9 10 11 12 13 /** * @version 1.5 2015-05-08 * @author Cay Horstmann */ public class CalendarTest { public static void main(String[] args) { LocalDate date = LocalDate.now(); int month = date.getMonthValue(); int today = date.getDayOfMonth(); 14 15 16 17 date = date.minusDays(today - 1); // set to start of month DayOfWeek weekday = date.getDayOfWeek(); int value = weekday.getValue(); // 1 = Monday, . . . , 7 = Sunday 18 19 System.out.println("Mon Tue Wed Thu Fri Sat Sun"); From the Library of Hristo Dimov Hristov 4.3 141 for (int i = 1; i < value; i++) System.out.print(" "); while (date.getMonthValue() == month) { System.out.printf("%3d", date.getDayOfMonth()); if (date.getDayOfMonth() == today) System.out.print("*"); else System.out.print(" "); date = date.plusDays(1); if (date.getDayOfWeek().getValue() == 1) System.out.println(); } if (date.getDayOfWeek().getValue() != 1) System.out.println(); 20 21 22 23 24 25 26 27 28 29 30 31 32 } 33 34 Defining Your Own Classes } java.time.LocalDate 8 • static LocalDate now() constructs an object that represents the current date. • static LocalDate of(int year, int month, int day) constructs an object that represents the given date. • int getYear() • int getMonthValue() • int getDayOfMonth() gets the year, month, and day of this date. • DayOfWeek getDayOfWeek gets the weekday of this date as an instance of the DayOfWeek class. Call getValue to get a weekday between 1 (Monday) and 7 (Sunday). • LocalDate plusDays(int n) • LocalDate minusDays(int n) yields the date that is n days after or before this date. 4.3 Defining Your Own Classes In Chapter 3, you started writing simple classes. However, all those classes had just a single main method. Now the time has come to show you how to write the kind of “workhorse classes” that are needed for more sophisticated applications. These classes typically do not have a main method. Instead, they have their own instance fields and methods. To build a complete program, you combine several classes, one of which has a main method. From the Library of Hristo Dimov Hristov 142 Chapter 4 Objects and Classes 4.3.1 An Employee Class The simplest form for a class definition in Java is class ClassName { field1 field2 . . . constructor1 constructor2 . . . method1 method2 . . . } Consider the following, very simplified, version of an Employee class that might be used by a business in writing a payroll system: class Employee { // instance fields private String name; private double salary; private LocalDate hireDay; // constructor public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; hireDay = LocalDate.of(year, month, day); } // a method public String getName() { return name; } // more methods . . . } From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 143 We break down the implementation of this class, in some detail, in the sections that follow. First, though, Listing 4.2 is a program that shows the Employee class in action. In the program, we construct an Employee array and fill it with three Employee objects: Employee[] staff = new Employee[3]; staff[0] = new Employee("Carl Cracker", . . .); staff[1] = new Employee("Harry Hacker", . . .); staff[2] = new Employee("Tony Tester", . . .); Next, we use the raiseSalary method of the Employee class to raise each employee’s salary by 5%: for (Employee e : staff) e.raiseSalary(5); Finally, we print out information about each employee, by calling the getName, getSalary, and getHireDay methods: for (Employee e : staff) System.out.println("name=" + e.getName() + ",salary=" + e.getSalary() + ",hireDay=" + e.getHireDay()); Note that the example program consists of two classes: the Employee class and a class EmployeeTest with the public access specifier. The main method with the instructions that we just described is contained in the EmployeeTest class. The name of the source file is EmployeeTest.java because the name of the file must match the name of the public class. You can only have one public class in a source file, but you can have any number of nonpublic classes. Next, when you compile this source code, the compiler creates two class files in the directory: EmployeeTest.class and Employee.class. You then start the program by giving the bytecode interpreter the name of the class that contains the main method of your program: java EmployeeTest The bytecode interpreter starts running the code in the main method in the class. This code in turn constructs three new Employee objects and shows you their state. EmployeeTest From the Library of Hristo Dimov Hristov 144 Chapter 4 Objects and Classes Listing 4.2 EmployeeTest/EmployeeTest.java 1 import java.time.*; 2 3 4 5 6 7 8 9 10 11 12 13 /** * This program tests the Employee class. * @version 1.13 2018-04-10 * @author Cay Horstmann */ public class EmployeeTest { public static void main(String[] args) { // fill the staff array with three Employee objects Employee[] staff = new Employee[3]; 14 staff[0] = new Employee("Carl Cracker", 75000, 1987, 12, 15); staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1); staff[2] = new Employee("Tony Tester", 40000, 1990, 3, 15); 15 16 17 18 // raise everyone's salary by 5% for (Employee e : staff) e.raiseSalary(5); 19 20 21 22 // print out information about all Employee objects for (Employee e : staff) System.out.println("name=" + e.getName() + ",salary=" + e.getSalary() + ",hireDay=" + e.getHireDay()); 23 24 25 26 } 27 28 } 29 30 31 32 33 34 class Employee { private String name; private double salary; private LocalDate hireDay; 35 36 37 38 39 40 41 public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; hireDay = LocalDate.of(year, month, day); } 42 43 44 45 46 public String getName() { return name; } 47 From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 145 public double getSalary() { return salary; } 48 49 50 51 52 public LocalDate getHireDay() { return hireDay; } 53 54 55 56 57 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 58 59 60 61 62 63 } 4.3.2 Use of Multiple Source Files The program in Listing 4.2 has two classes in a single source file. Many programmers prefer to put each class into its own source file. For example, you can place the Employee class into a file Employee.java and the EmployeeTest class into EmployeeTest.java. If you like this arrangement, you have two choices for compiling the program. You can invoke the Java compiler with a wildcard: javac Employee*.java Then, all source files matching the wildcard will be compiled into class files. Or, you can simply type javac EmployeeTest.java You may find it surprising that the second choice works even though the Employee.java file is never explicitly compiled. However, when the Java compiler sees the Employee class being used inside EmployeeTest.java, it will look for a file named Employee.class. If it does not find that file, it automatically searches for Employee.java and compiles it. Moreover, if the timestamp of the version of Employee.java that it finds is newer than that of the existing Employee.class file, the Java compiler will automatically recompile the file. NOTE: If you are familiar with the make facility of UNIX (or one of its Windows cousins, such as nmake), you can think of the Java compiler as having the make functionality already built in. From the Library of Hristo Dimov Hristov 146 Chapter 4 Objects and Classes 4.3.3 Dissecting the Employee Class In the sections that follow, we will dissect the Employee class. Let’s start with the methods in this class. As you can see by examining the source code, this class has one constructor and four methods: public public public public public Employee(String n, double s, int year, int month, int day) String getName() double getSalary() LocalDate getHireDay() void raiseSalary(double byPercent) All methods of this class are tagged as public. The keyword public means that any method in any class can call the method. (The four possible access levels are covered in this and the next chapter.) Next, notice the three instance fields that will hold the data manipulated inside an instance of the Employee class. private String name; private double salary; private LocalDate hireDay; The private keyword makes sure that the only methods that can access these instance fields are the methods of the Employee class itself. No outside method can read or write to these fields. NOTE: You could use the public keyword with your instance fields, but it would be a very bad idea. Having public data fields would allow any part of the program to read and modify the instance fields, completely ruining encapsulation. Any method of any class can modify public fields—and, in our experience, some code will take advantage of that access privilege when you least expect it. We strongly recommend to make all your instance fields private. Finally, notice that two of the instance fields are themselves objects: The name and hireDay fields are references to String and LocalDate objects. This is quite usual: Classes will often contain instance fields of class type. 4.3.4 First Steps with Constructors Let’s look at the constructor listed in our Employee class. public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 147 hireDay = LocalDate.of(year, month, day); } As you can see, the name of the constructor is the same as the name of the class. This constructor runs when you construct objects of the Employee class—giving the instance fields the initial state you want them to have. For example, when you create an instance of the Employee class with code like this: new Employee("James Bond", 100000, 1950, 1, 1) you have set the instance fields as follows: name = "James Bond"; salary = 100000; hireDay = LocalDate.of(1950, 1, 1); // January 1, 1950 There is an important difference between constructors and other methods. A constructor can only be called in conjunction with the new operator. You can’t apply a constructor to an existing object to reset the instance fields. For example, james.Employee("James Bond", 250000, 1950, 1, 1) // ERROR is a compile-time error. We will have more to say about constructors later in this chapter. For now, keep the following in mind: • • • • • A A A A A constructor has the same name as the class. class can have more than one constructor. constructor can take zero, one, or more parameters. constructor has no return value. constructor is always called with the new operator. C++ NOTE: Constructors work the same way in Java as they do in C++. Keep in mind, however, that all Java objects are constructed on the heap and that a constructor must be combined with new. It is a common error of C++ programmers to forget the new operator: Employee number007("James Bond", 100000, 1950, 1, 1); // C++, not Java That works in C++ but not in Java. From the Library of Hristo Dimov Hristov 148 Chapter 4 Objects and Classes CAUTION: Be careful not to introduce local variables with the same names as the instance fields. For example, the following constructor will not set the salary: public Employee(String n, double s, . . .) { String name = n; // ERROR double salary = s; // ERROR . . . } The constructor declares local variables name and salary. These variables are only accessible inside the constructor. They shadow the instance fields with the same name. Some programmers accidentally write this kind of code when they type faster than they think, because their fingers are used to adding the data type. This is a nasty error that can be hard to track down. You just have to be careful in all of your methods to not use variable names that equal the names of instance fields. 4.3.5 Declaring Local Variables with var As of Java 10, you can declare local variables with the var keyword instead of specifying their type, provided their type can be inferred from the initial value. For example, instead of declaring Employee harry = new Employee("Harry Hacker", 50000, 1989, 10, 1); you simply write var harry = new Employee("Harry Hacker", 50000, 1989, 10, 1); This is nice since it avoids the repetition of the type name Employee. From now on, we will use the var notation in those cases where the type is obvious from the right-hand side without any knowledge of the Java API. But we won’t use var with numeric types such as int, long, or double so that you don’t have to look out for the difference between 0, 0L, and 0.0. Once you are more experienced with the Java API, you may want to use the var keyword more frequently. Note that the var keyword can only be used with local variables inside methods. You must always declare the types of parameters and fields. 4.3.6 Working with null References In Section 4.2.1, “Objects and Object Variables,” on p. 132, you saw that an object variable holds a reference to an object, or the special value null to indicate the absence of an object. From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 149 This sounds like a convenient mechanism for dealing with special situations, such as an unknown name or hire date. But you need to be very careful with null values. If you apply a method to a null value, a NullPointerException occurs. LocalDate birthday = null; String s = birthday.toString(); // NullPointerException This is a serious error, similar to an “index out of bounds” exception. If your program does not “catch” an exception, it is terminated. Normally, programs don’t catch these kinds of exceptions but rely on programmers not to cause them in the first place. When you define a class, it is a good idea to be clear about which fields can be null. In our example, we don’t want the name or hireDay field to be null. (We don’t have to worry about the salary field. It has primitive type and can never be null.) The hireDay field is guaranteed to be non-null because it is initialized with a new LocalDate object. But name will be null if the constructor is called with a null argument for n. There are two solutions. The “permissive” approach is to turn a null argument into an appropriate non-null value: if (n == null) name = "unknown"; else name = n; As of Java 9, the Objects class has a convenience method for this purpose: public Employee(String n, double s, int year, int month, int day) { name = Objects.requireNonNullElse(n, "unknown"); . . . } The “tough love” approach is to reject a null argument: public Employee(String n, double s, int year, int month, int day) { Objects.requireNonNull(n, "The name cannot be null"); name = n; . . . } If someone constructs an Employee object with a null name, then a NullPointerException occurs. At first glance, that may not seem a useful remedy. But there are two advantages: From the Library of Hristo Dimov Hristov 150 Chapter 4 1. 2. Objects and Classes The exception report has a description of the problem. The exception report pinpoints the location of the problem. Otherwise, a NullPointerException would have occurred elsewhere, with no easy way of tracing it back to the faulty constructor argument. NOTE: Whenever you accept an object reference as a construction parameter, ask yourself whether you really intend to model values that can be present or absent. If not, the “tough love” approach is preferred. 4.3.7 Implicit and Explicit Parameters Methods operate on objects and access their instance fields. For example, the method public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } sets a new value for the salary instance field in the object on which this method is invoked. Consider the call number007.raiseSalary(5); The effect is to increase the value of the number007.salary field by 5%. More specifically, the call executes the following instructions: double raise = number007.salary * 5 / 100; number007.salary += raise; The raiseSalary method has two parameters. The first parameter, called the implicit parameter, is the object of type Employee that appears before the method name. The second parameter, the number inside the parentheses after the method name, is an explicit parameter. (Some people call the implicit parameter the target or receiver of the method call.) As you can see, the explicit parameters are explicitly listed in the method declaration—for example, double byPercent. The implicit parameter does not appear in the method declaration. In every method, the keyword this refers to the implicit parameter. If you like, you can write the raiseSalary method as follows: From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 151 public void raiseSalary(double byPercent) { double raise = this.salary * byPercent / 100; this.salary += raise; } Some programmers prefer that style because it clearly distinguishes between instance fields and local variables. C++ NOTE: In C++, you generally define methods outside the class: void Employee::raiseSalary(double byPercent) // C++, not Java { . . . } If you define a method inside a class, then it is, automatically, an inline method. class Employee { . . . int getName() { return name; } // inline in C++ } In Java, all methods are defined inside the class itself. This does not make them inline. Finding opportunities for inline replacement is the job of the Java virtual machine. The just-in-time compiler watches for calls to methods that are short, commonly called, and not overridden, and optimizes them away. 4.3.8 Benefits of Encapsulation Finally, let’s look more closely at the rather simple getName, getSalary, and getHireDay methods. public String getName() { return name; } public double getSalary() { return salary; } public LocalDate getHireDay() { return hireDay; } From the Library of Hristo Dimov Hristov 152 Chapter 4 Objects and Classes These are obvious examples of accessor methods. As they simply return the values of instance fields, they are sometimes called field accessors. Wouldn’t it be easier to make the name, salary, and hireDay fields public, instead of having separate accessor methods? However, the name field is read-only. Once you set it in the constructor, there is no method to change it. Thus, we have a guarantee that the name field will never be corrupted. The salary field is not read-only, but it can only be changed by the raiseSalary method. In particular, should the value ever turn out wrong, only that method needs to be debugged. Had the salary field been public, the culprit for messing up the value could have been anywhere. Sometimes, it happens that you want to get and set the value of an instance field. Then you need to supply three items: • A private data field; • A public field accessor method; and • A public field mutator method. This is a lot more tedious than supplying a single public data field, but there are considerable benefits. First, you can change the internal implementation without affecting any code other than the methods of the class. For example, if the storage of the name is changed to String firstName; String lastName; then the getName method can be changed to return firstName + " " + lastName This change is completely invisible to the remainder of the program. Of course, the accessor and mutator methods may need to do a lot of work to convert between the old and the new data representation. That leads us to our second benefit: Mutator methods can perform error checking, whereas code that simply assigns to a field may not go into the trouble. For example, a setSalary method might check that the salary is never less than 0. From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 153 CAUTION: Be careful not to write accessor methods that return references to mutable objects. In a previous edition of this book, we violated that rule in our Employee class in which the getHireDay method returned an object of class Date: class Employee { private Date hireDay; . . . public Date getHireDay() { return hireDay; // BAD } . . . } Unlike the LocalDate class, which has no mutator methods, the Date class has a mutator method, setTime, where you can set the number of milliseconds. The fact that Date objects are mutable breaks encapsulation! Consider the following rogue code: Employee harry = . . .; Date d = harry.getHireDay(); double tenYearsInMilliSeconds = 10 * 365.25 * 24 * 60 * 60 * 1000; d.setTime(d.getTime() - (long) tenYearsInMilliSeconds); // let's give Harry ten years of added seniority The reason is subtle. Both d and harry.hireDay refer to the same object (see Figure 4.5). Applying mutator methods to d automatically changes the private state of the Employee object! If you need to return a reference to a mutable object, you should clone it first. A clone is an exact copy of an object stored in a new location. We discuss cloning in detail in Chapter 6. Here is the corrected code: class Employee { . . . public Date getHireDay() { return (Date) hireDay.clone(); // OK } . . . } As a rule of thumb, always use clone whenever you need to return a copy of a mutable field. From the Library of Hristo Dimov Hristov 154 Chapter 4 Objects and Classes Figure 4.5 Returning a reference to a mutable data field 4.3.9 Class-Based Access Privileges You know that a method can access the private data of the object on which it is invoked. What people often find surprising is that a method can access the private data of all objects of its class. For example, consider a method equals that compares two employees. class Employee { . . . public boolean equals(Employee other) { return name.equals(other.name); } } A typical call is if (harry.equals(boss)) . . . This method accesses the private fields of harry, which is not surprising. It also accesses the private fields of boss. This is legal because boss is an object of type Employee, and a method of the Employee class is permitted to access the private fields of any object of type Employee. C++ NOTE: C++ has the same rule. A method can access the private features of any object of its class, not just of the implicit parameter. From the Library of Hristo Dimov Hristov 4.3 Defining Your Own Classes 155 4.3.10 Private Methods When implementing a class, we make all data fields private because public data are dangerous. But what about the methods? While most methods are public, private methods are useful in certain circumstances. Sometimes, you may wish to break up the code for a computation into separate helper methods. Typically, these helper methods should not be part of the public interface—they may be too close to the current implementation or require a special protocol or calling order. Such methods are best implemented as private. To implement a private method in Java, simply change the public keyword to private. By making a method private, you are under no obligation to keep it available if you change your implementation. The method may well be harder to implement or unnecessary if the data representation changes; this is irrelevant. The point is that as long as the method is private, the designers of the class can be assured that it is never used elsewhere, so they can simply drop it. If a method is public, you cannot simply drop it because other code might rely on it. 4.3.11 Final Instance Fields You can define an instance field as final. Such a field must be initialized when the object is constructed. That is, you must guarantee that the field value has been set after the end of every constructor. Afterwards, the field may not be modified again. For example, the name field of the Employee class may be declared as final because it never changes after the object is constructed—there is no setName method. class Employee { private final String name; . . . } The final modifier is particularly useful for fields whose type is primitive or an immutable class. (A class is immutable if none of its methods ever mutate its objects. For example, the String class is immutable.) For mutable classes, the final modifier can be confusing. For example, consider a field private final StringBuilder evaluations; From the Library of Hristo Dimov Hristov 156 Chapter 4 Objects and Classes that is initialized in the Employee constructor as evaluations = new StringBuilder(); The final keyword merely means that the object reference stored in the evaluations variable will never again refer to a different StringBuilder object. But the object can be mutated: public void giveGoldStar() { evaluations.append(LocalDate.now() + ": Gold star!\n"); } 4.4 Static Fields and Methods In all sample programs that you have seen, the main method is tagged with the static modifier. We are now ready to discuss the meaning of this modifier. 4.4.1 Static Fields If you define a field as static, then there is only one such field per class. In contrast, each object has its own copy of nonstatic instance fields. For example, let’s suppose we want to assign a unique identification number to each employee. We add an instance field id and a static field nextId to the Employee class: class Employee { private static int nextId = 1; private int id; . . . } Every Employee object now has its own id field, but there is only one nextId field that is shared among all instances of the class. Let’s put it another way. If there are 1,000 objects of the Employee class, then there are 1,000 instance fields id, one for each object. But there is a single static field nextId. Even if there are no Employee objects, the static field nextId is present. It belongs to the class, not to any individual object. NOTE: In some object-oriented programming languages, static fields are called class fields. The term “static” is a meaningless holdover from C++. From the Library of Hristo Dimov Hristov 4.4 Static Fields and Methods 157 Let’s implement a simple method: public void setId() { id = nextId; nextId++; } Suppose you set the employee identification number for harry: harry.setId(); Then, the id field of harry is set to the current value of the static field nextId, and the value of the static field is incremented: harry.id = Employee.nextId; Employee.nextId++; 4.4.2 Static Constants Static variables are quite rare. However, static constants are more common. For example, the Math class defines a static constant: public class Math { . . . public static final double PI = 3.14159265358979323846; . . . } You can access this constant in your programs as Math.PI. If the keyword static had been omitted, then PI would have been an instance field of the Math class. That is, you would need an object of this class to access PI, and every Math object would have its own copy of PI. Another static constant that you have used many times is System.out. It is declared in the System class as follows: public class System { . . . public static final PrintStream out = . . .; . . . } As we mentioned several times, it is never a good idea to have public fields, because everyone can modify them. However, public constants (that is, final From the Library of Hristo Dimov Hristov 158 Chapter 4 Objects and Classes fields) are fine. Since out has been declared as final, you cannot reassign another print stream to it: System.out = new PrintStream(. . .); // ERROR--out is final NOTE: If you look at the System class, you will notice a method setOut that sets System.out to a different stream. You may wonder how that method can change the value of a final variable. However, the setOut method is a native method, not implemented in the Java programming language. Native methods can bypass the access control mechanisms of the Java language. This is a very unusual workaround that you should not emulate in your programs. 4.4.3 Static Methods Static methods are methods that do not operate on objects. For example, the pow method of the Math class is a static method. The expression Math.pow(x, a) computes the power xa. It does not use any Math object to carry out its task. In other words, it has no implicit parameter. You can think of static methods as methods that don’t have a this parameter. (In a nonstatic method, the this parameter refers to the implicit parameter of the method—see Section 4.3.7, “Implicit and Explicit Parameters,” on p. 150.) A static method of the Employee class cannot access the id instance field because it does not operate on an object. However, a static method can access a static field. Here is an example of such a static method: public static int getNextId() { return nextId; // returns static field } To call this method, you supply the name of the class: int n = Employee.getNextId(); Could you have omitted the keyword static for this method? Yes, but then you would need to have an object reference of type Employee to invoke the method. From the Library of Hristo Dimov Hristov 4.4 Static Fields and Methods 159 NOTE: It is legal to use an object to call a static method. For example, if harry is an Employee object, then you can call harry.getNextId() instead of Employee.getNextId(). However, we find that notation confusing. The getNextId method doesn’t look at harry at all to compute the result. We recommend that you use class names, not objects, to invoke static methods. Use static methods in two situations: • When a method doesn’t need to access the object state because all needed parameters are supplied as explicit parameters (example: Math.pow). • When a method only needs to access static fields of the class (example: Employee.getNextId). C++ NOTE: Static fields and methods have the same functionality in Java and C++. However, the syntax is slightly different. In C++, you use the :: operator to access a static field or method outside its scope, such as Math::PI. The term “static” has a curious history. At first, the keyword static was introduced in C to denote local variables that don’t go away when a block is exited. In that context, the term “static” makes sense: The variable stays around and is still there when the block is entered again. Then static got a second meaning in C, to denote global variables and functions that cannot be accessed from other files. The keyword static was simply reused to avoid introducing a new keyword. Finally, C++ reused the keyword for a third, unrelated, interpretation—to denote variables and functions that belong to a class but not to any particular object of the class. That is the same meaning the keyword has in Java. 4.4.4 Factory Methods Here is another common use for static methods. Classes such as LocalDate and NumberFormat use static factory methods that construct objects. You have already seen the factory methods LocalDate.now and LocalDate.of. Here is how the NumberFormat class yields formatter objects for various styles: NumberFormat currencyFormatter = NumberFormat.getCurrencyInstance(); NumberFormat percentFormatter = NumberFormat.getPercentInstance(); double x = 0.1; System.out.println(currencyFormatter.format(x)); // prints $0.10 System.out.println(percentFormatter.format(x)); // prints 10% From the Library of Hristo Dimov Hristov 160 Chapter 4 Objects and Classes Why doesn’t the NumberFormat class use a constructor instead? There are two reasons: • You can’t give names to constructors. The constructor name is always the same as the class name. But we want two different names to get the currency instance and the percent instance. • When you use a constructor, you can’t vary the type of the constructed object. But the factory methods actually return objects of the class DecimalFormat, a subclass that inherits from NumberFormat. (See Chapter 5 for more on inheritance.) 4.4.5 The main Method Note that you can call static methods without having any objects. For example, you never construct any objects of the Math class to call Math.pow. For the same reason, the main method is a static method. public class Application { public static void main(String[] args) { // construct objects here . . . } } The main method does not operate on any objects. In fact, when a program starts, there aren’t any objects yet. The static main method executes, and constructs the objects that the program needs. TIP: Every class can have a main method. That is a handy trick for unit testing of classes. For example, you can add a main method to the Employee class: class Employee { public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; hireDay = LocalDate.of(year, month, day); } . . . public static void main(String[] args) // unit test { From the Library of Hristo Dimov Hristov 4.4 Static Fields and Methods 161 var e = new Employee("Romeo", 50000, 2003, 3, 31); e.raiseSalary(10); System.out.println(e.getName() + " " + e.getSalary()); } . . . } If you want to test the Employee class in isolation, simply execute java Employee If the Employee class is a part of a larger application, you start the application with java Application and the main method of the Employee class is never executed. The program in Listing 4.3 contains a simple version of the Employee class with a static field nextId and a static method getNextId. We fill an array with three Employee objects and then print the employee information. Finally, we print the next available identification number, to demonstrate the static method. Note that the Employee class also has a static main method for unit testing. Try running both java Employee and java StaticTest to execute both main methods. Listing 4.3 StaticTest/StaticTest.java 1 2 3 4 5 6 7 8 9 10 11 /** * This program demonstrates static methods. * @version 1.02 2008-04-10 * @author Cay Horstmann */ public class StaticTest { public static void main(String[] args) { // fill the staff array with three Employee objects var staff = new Employee[3]; 12 (Continues) From the Library of Hristo Dimov Hristov 162 Chapter 4 Objects and Classes Listing 4.3 (Continued) staff[0] = new Employee("Tom", 40000); staff[1] = new Employee("Dick", 60000); staff[2] = new Employee("Harry", 65000); 13 14 15 16 // print out information about all Employee objects for (Employee e : staff) { e.setId(); System.out.println("name=" + e.getName() + ",id=" + e.getId() + ",salary=" + e.getSalary()); } 17 18 19 20 21 22 23 24 int n = Employee.getNextId(); // calls static method System.out.println("Next available id=" + n); 25 26 } 27 28 } 29 30 31 32 class Employee { private static int nextId = 1; 33 34 35 36 private String name; private double salary; private int id; 37 38 39 40 41 42 43 public Employee(String n, double s) { name = n; salary = s; id = 0; } 44 45 46 47 48 public String getName() { return name; } 49 50 51 52 53 public double getSalary() { return salary; } 54 55 56 57 58 public int getId() { return id; } 59 From the Library of Hristo Dimov Hristov 4.5 Method Parameters 163 public void setId() { id = nextId; // set id to next available id nextId++; } 60 61 62 63 64 65 public static int getNextId() { return nextId; // returns static field } 66 67 68 69 70 public static void main(String[] args) // unit test { var e = new Employee("Harry", 50000); System.out.println(e.getName() + " " + e.getSalary()); } 71 72 73 74 75 76 } java.util.Objects 7 • static <T> void requireNonNull(T obj) • static <T> void requireNonNull(T obj, String message) • static <T> void requireNonNull(T obj, Supplier<String> messageSupplier) 8 If obj is null, these methods throw a NullPointerException with no message or the given message. (Chapter 6 explains how to obtain a value lazily with a supplier. Chapter 8 explains the <T> syntax.) • static <T> T requireNonNullElse(T obj, T defaultObj) • static <T> T requireNonNullElseGet(T obj, Supplier<T> defaultSupplier) Returns obj if it is not null, or the default object if obj is null. 4.5 Method Parameters Let us review the computer science terms that describe how parameters can be passed to a method (or a function) in a programming language. The term call by value means that the method gets just the value that the caller provides. In contrast, call by reference means that the method gets the location of the variable that the caller provides. Thus, a method can modify the value stored in a variable passed by reference but not in one passed by value. These “call by . . .” terms are standard computer science terminology describing the behavior of method parameters in various programming languages, not just Java. (There is also a call by name that is mainly of historical interest, being employed in the Algol programming language, one of the oldest high-level languages.) From the Library of Hristo Dimov Hristov 164 Chapter 4 Objects and Classes The Java programming language always uses call by value. That means that the method gets a copy of all parameter values. In particular, the method cannot modify the contents of any parameter variables passed to it. For example, consider the following call: double percent = 10; harry.raiseSalary(percent); No matter how the method is implemented, we know that after the method call, the value of percent is still 10. Let us look a little more closely at this situation. Suppose a method tried to triple the value of a method parameter: public static void tripleValue(double x) // doesn't work { x = 3 * x; } Let’s call this method: double percent = 10; tripleValue(percent); However, this does not work. After the method call, the value of percent is still 10. Here is what happens: 1. 2. 3. is initialized with a copy of the value of percent (that is, 10). x is tripled—it is now 30. But percent is still 10 (see Figure 4.6). The method ends, and the parameter variable x is no longer in use. x There are, however, two kinds of method parameters: • Primitive types (numbers, boolean values) • Object references You have seen that it is impossible for a method to change a primitive type parameter. The situation is different for object parameters. You can easily implement a method that triples the salary of an employee: public static void tripleSalary(Employee x) // works { x.raiseSalary(200); } When you call harry = new Employee(. . .); tripleSalary(harry); From the Library of Hristo Dimov Hristov 4.5 Method Parameters 165 Figure 4.6 Modifying a numeric parameter has no lasting effect. then the following happens: 1. 2. 3. is initialized with a copy of the value of harry—that is, an object reference. The raiseSalary method is applied to that object reference. The Employee object to which both x and harry refer gets its salary raised by 200 percent. The method ends, and the parameter variable x is no longer in use. Of course, the object variable harry continues to refer to the object whose salary was tripled (see Figure 4.7). x As you have seen, it is easily possible—and in fact very common—to implement methods that change the state of an object parameter. The reason is simple. The method gets a copy of the object reference, and both the original and the copy refer to the same object. Many programming languages (in particular, C++ and Pascal) have two mechanisms for parameter passing: call by value and call by reference. Some programmers (and unfortunately even some book authors) claim that Java uses call by reference for objects. That is false. As this is such a common misunderstanding, it is worth examining a counterexample in detail. From the Library of Hristo Dimov Hristov 166 Chapter 4 Objects and Classes Figure 4.7 Modifying an object parameter has a lasting effect. Let’s try to write a method that swaps two Employee objects: public static void swap(Employee x, Employee y) // doesn't work { Employee temp = x; x = y; y = temp; } If Java used call by reference for objects, this method would work: var a = var b = swap(a, // does new Employee("Alice", . . .); new Employee("Bob", . . .); b); a now refer to Bob, b to Alice? However, the method does not actually change the object references that are stored in the variables a and b. The x and y parameters of the swap method are initialized with copies of these references. The method then proceeds to swap these copies. // x refers to Alice, y to Bob Employee temp = x; x = y; y = temp; // now x refers to Bob, y to Alice From the Library of Hristo Dimov Hristov 4.5 Method Parameters 167 But ultimately, this is a wasted effort. When the method ends, the parameter variables x and y are abandoned. The original variables a and b still refer to the same objects as they did before the method call (see Figure 4.8). Figure 4.8 Swapping object parameters has no lasting effect. This demonstrates that the Java programming language does not use call by reference for objects. Instead, object references are passed by value. Here is a summary of what you can and cannot do with method parameters in Java: • A method cannot modify a parameter of a primitive type (that is, numbers or boolean values). • A method can change the state of an object parameter. • A method cannot make an object parameter refer to a new object. The program in Listing 4.4 demonstrates these facts. The program first tries to triple the value of a number parameter and does not succeed: Testing tripleValue: Before: percent=10.0 End of method: x=30.0 After: percent=10.0 It then successfully triples the salary of an employee: From the Library of Hristo Dimov Hristov 168 Chapter 4 Objects and Classes Testing tripleSalary: Before: salary=50000.0 End of method: salary=150000.0 After: salary=150000.0 After the method, the state of the object to which harry refers has changed. This is possible because the method modified the state through a copy of the object reference. Finally, the program demonstrates the failure of the swap method: Testing swap: Before: a=Alice Before: b=Bob End of method: x=Bob End of method: y=Alice After: a=Alice After: b=Bob As you can see, the parameter variables x and y are swapped, but the variables a and b are not affected. C++ NOTE: C++ has both call by value and call by reference. You tag reference parameters with &. For example, you can easily implement methods void tripleValue(double& x) or void swap(Employee& x, Employee& y) that modify their reference parameters. Listing 4.4 ParamTest/ParamTest.java 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 /** * This program demonstrates parameter passing in Java. * @version 1.01 2018-04-10 * @author Cay Horstmann */ public class ParamTest { public static void main(String[] args) { /* * Test 1: Methods can't modify numeric parameters */ System.out.println("Testing tripleValue:"); double percent = 10; System.out.println("Before: percent=" + percent); tripleValue(percent); System.out.println("After: percent=" + percent); 18 From the Library of Hristo Dimov Hristov 4.5 Method Parameters 169 /* * Test 2: Methods can change the state of object parameters */ System.out.println("\nTesting tripleSalary:"); var harry = new Employee("Harry", 50000); System.out.println("Before: salary=" + harry.getSalary()); tripleSalary(harry); System.out.println("After: salary=" + harry.getSalary()); 19 20 21 22 23 24 25 26 27 /* * Test 3: Methods can't attach new objects to object parameters */ System.out.println("\nTesting swap:"); var a = new Employee("Alice", 70000); var b = new Employee("Bob", 60000); System.out.println("Before: a=" + a.getName()); System.out.println("Before: b=" + b.getName()); swap(a, b); System.out.println("After: a=" + a.getName()); System.out.println("After: b=" + b.getName()); 28 29 30 31 32 33 34 35 36 37 38 } 39 40 public static void tripleValue(double x) // doesn't work { x = 3 * x; System.out.println("End of method: x=" + x); } 41 42 43 44 45 46 public static void tripleSalary(Employee x) // works { x.raiseSalary(200); System.out.println("End of method: salary=" + x.getSalary()); } 47 48 49 50 51 52 public static void swap(Employee x, Employee y) { Employee temp = x; x = y; y = temp; System.out.println("End of method: x=" + x.getName()); System.out.println("End of method: y=" + y.getName()); } 53 54 55 56 57 58 59 60 61 } 62 63 64 65 66 class Employee // simplified Employee class { private String name; private double salary; 67 (Continues) From the Library of Hristo Dimov Hristov 170 Chapter 4 Objects and Classes Listing 4.4 (Continued) public Employee(String n, double s) { name = n; salary = s; } 68 69 70 71 72 73 public String getName() { return name; } 74 75 76 77 78 public double getSalary() { return salary; } 79 80 81 82 83 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 84 85 86 87 88 89 } 4.6 Object Construction You have seen how to write simple constructors that define the initial state of your objects. However, since object construction is so important, Java offers quite a variety of mechanisms for writing constructors. We go over these mechanisms in the sections that follow. 4.6.1 Overloading Some classes have more than one constructor. For example, you can construct an empty StringBuilder object as var messages = new StringBuilder(); Alternatively, you can specify an initial string: var todoList = new StringBuilder("To do:\n"); This capability is called overloading. Overloading occurs if several methods have the same name (in this case, the StringBuilder constructor method) but different parameters. The compiler must sort out which method to call. It From the Library of Hristo Dimov Hristov 4.6 Object Construction 171 picks the correct method by matching the parameter types in the headers of the various methods with the types of the values used in the specific method call. A compile-time error occurs if the compiler cannot match the parameters, either because there is no match at all or because there is not one that is better than all others. (The process of finding a match is called overloading resolution.) NOTE: Java allows you to overload any method—not just constructor methods. Thus, to completely describe a method, you need to specify its name together with its parameter types. This is called the signature of the method. For example, the String class has four public methods called indexOf. They have signatures indexOf(int) indexOf(int, int) indexOf(String) indexOf(String, int) The return type is not part of the method signature. That is, you cannot have two methods with the same names and parameter types but different return types. 4.6.2 Default Field Initialization If you don’t set a field explicitly in a constructor, it is automatically set to a default value: numbers to 0, boolean values to false, and object references to null. Some people consider it poor programming practice to rely on the defaults. Certainly, it makes it harder for someone to understand your code if fields are being initialized invisibly. NOTE: This is an important difference between fields and local variables. You must always explicitly initialize local variables in a method. But in a class, if you don’t initialize a field, it is automatically initialized to a default (0, false, or null). For example, consider the Employee class. Suppose you don’t specify how to initialize some of the fields in a constructor. By default, the salary field would be initialized with 0 and the name and hireDay fields would be initialized with null. However, that would not be a good idea. If anyone called the getName or getHireDay method, they would get a null reference that they probably don’t expect: LocalDate h = harry.getHireDay(); int year = h.getYear(); // throws exception if h is null From the Library of Hristo Dimov Hristov 172 Chapter 4 Objects and Classes 4.6.3 The Constructor with No Arguments Many classes contain a constructor with no arguments that creates an object whose state is set to an appropriate default. For example, here is a no-argument constructor for the Employee class: public Employee() { name = ""; salary = 0; hireDay = LocalDate.now(); } If you write a class with no constructors whatsoever, then a no-argument constructor is provided for you. This constructor sets all the instance fields to their default values. So, all numeric data contained in the instance fields would be 0, all boolean values would be false, and all object variables would be null. If a class supplies at least one constructor but does not supply a no-argument constructor, it is illegal to construct objects without supplying arguments. For example, our original Employee class in Listing 4.2 provided a single constructor: public Employee(String n, double s, int year, int month, int day) With that class, it was not legal to construct default employees. That is, the call e = new Employee(); would have been an error. CAUTION: Please keep in mind that you get a free no-argument constructor only when your class has no other constructors. If you write your class with even a single constructor of your own and you want the users of your class to have the ability to create an instance by a call to new ClassName() then you must provide a no-argument constructor. Of course, if you are happy with the default values for all fields, you can simply supply public ClassName() { } From the Library of Hristo Dimov Hristov 4.6 Object Construction 173 C++ NOTE: C++ uses the “initializer list” syntax for constructing fields: Employee::Employee(String n, double s, int y, int m, int d) // C++ : name(n), salary(s), hireDay(y, m, d) { } C++ uses this special syntax to avoid unnecessary invocations of no-argument constructors. In Java, there is no need for that because objects have no subobjects, only pointers to other objects. 4.6.4 Explicit Field Initialization By overloading the constructor methods in a class, you can build many ways to set the initial state of the instance fields of your classes. It is always a good idea to make sure that, regardless of the constructor call, every instance field is set to something meaningful. You can simply assign a value to any field in the class definition. For example: class Employee { private String name = ""; . . . } This assignment is carried out before the constructor executes. This syntax is particularly useful if all constructors of a class need to set a particular instance field to the same value. The initialization value doesn’t have to be a constant value. Here is an example in which a field is initialized with a method call. Consider an Employee class where each employee has an id field. You can initialize it as follows: class Employee { private static int nextId; private int id = assignId(); . . . private static int assignId() { int r = nextId; nextId++; return r; } . . . } From the Library of Hristo Dimov Hristov 174 Chapter 4 Objects and Classes 4.6.5 Parameter Names When you write very trivial constructors (and you’ll write a lot of them), it can be somewhat frustrating to come up with parameter names. We have generally opted for single-letter parameter names: public Employee(String n, double s) { name = n; salary = s; } However, the drawback is that you need to read the code to tell what the n and s parameters mean. Some programmers prefix each parameter with an “a”: public Employee(String aName, double aSalary) { name = aName; salary = aSalary; } That is quite neat. Any reader can immediately figure out the meaning of the parameters. Another commonly used trick relies on the fact that parameter variables shadow instance fields with the same name. For example, if you call a parameter salary, then salary refers to the parameter, not the instance field. But you can still access the instance field as this.salary. Recall that this denotes the implicit parameter—that is, the object being constructed. Here is an example: public Employee(String name, double salary) { this.name = name; this.salary = salary; } C++ NOTE: In C++, it is common to prefix instance fields with an underscore or a fixed letter. (The letters m and x are common choices.) For example, the salary field might be called _salary, mSalary, or xSalary. Java programmers don’t usually do that. From the Library of Hristo Dimov Hristov 4.6 Object Construction 175 4.6.6 Calling Another Constructor The keyword this refers to the implicit parameter of a method. However, this keyword has a second meaning. If the first statement of a constructor has the form this(. . .), then the constructor calls another constructor of the same class. Here is a typical example: public Employee(double s) { // calls Employee(String, double) this("Employee #" + nextId, s); nextId++; } When you call new Employee(60000), the Employee(double) constructor calls the Employee(String, double) constructor. Using the this keyword in this manner is useful—you only need to write common construction code once. C++ NOTE: The this reference in Java is identical to the this pointer in C++. However, in C++ it is not possible for one constructor to call another. If you want to factor out common initialization code in C++, you must write a separate method. 4.6.7 Initialization Blocks You have already seen two ways to initialize a data field: • By setting a value in a constructor • By assigning a value in the declaration There is a third mechanism in Java, called an initialization block. Class declarations can contain arbitrary blocks of code. These blocks are executed whenever an object of that class is constructed. For example: class Employee { private static int nextId; private int id; private String name; private double salary; // object initialization block { From the Library of Hristo Dimov Hristov 176 Chapter 4 Objects and Classes id = nextId; nextId++; } public Employee(String n, double s) { name = n; salary = s; } public Employee() { name = ""; salary = 0; } . . . } In this example, the id field is initialized in the object initialization block, no matter which constructor is used to construct an object. The initialization block runs first, and then the body of the constructor is executed. This mechanism is never necessary and is not common. It is usually more straightforward to place the initialization code inside a constructor. NOTE: It is legal to set fields in initialization blocks even if they are only defined later in the class. However, to avoid circular definitions, it is not legal to read from fields that are only initialized later. The exact rules are spelled out in Section 8.3.2.3 of the Java Language Specification (http://docs.oracle.com /javase/specs). The rules are complex enough to baffle the compiler implementors—early versions of Java implemented them with subtle errors. Therefore, we suggest that you always place initialization blocks after the field definitions. With so many ways of initializing data fields, it can be quite confusing to give all possible pathways for the construction process. Here is what happens in detail when a constructor is called: 1. 2. If the first line of the constructor calls a second constructor, then the second constructor executes with the provided arguments. Otherwise, a) b) All data fields are initialized to their default values (0, false, or null). All field initializers and initialization blocks are executed, in the order in which they occur in the class declaration. From the Library of Hristo Dimov Hristov 4.6 3. Object Construction 177 The body of the constructor is executed. Naturally, it is always a good idea to organize your initialization code so that another programmer could easily understand it without having to be a language lawyer. For example, it would be quite strange and somewhat errorprone to have a class whose constructors depend on the order in which the data fields are declared. To initialize a static field, either supply an initial value or use a static initialization block. You have already seen the first mechanism: private static int nextId = 1; If the static fields of your class require complex initialization code, use a static initialization block. Place the code inside a block and tag it with the keyword static. Here is an example. We want the employee ID numbers to start at a random integer less than 10,000. // static initialization block static { var generator = new Random(); nextId = generator.nextInt(10000); } Static initialization occurs when the class is first loaded. Like instance fields, static fields are 0, false, or null unless you explicitly set them to another value. All static field initializers and static initialization blocks are executed in the order in which they occur in the class declaration. NOTE: Amazingly enough, up to JDK 6, it was possible to write a “Hello, World” program in Java without ever writing a main method. public class Hello { static { System.out.println("Hello, World"); } } When you invoked the class with java Hello, the class was loaded, the static initialization block printed “Hello, World”, and only then was a message displayed that main is not defined. Since Java 7, the java program first checks that there is a main method. From the Library of Hristo Dimov Hristov 178 Chapter 4 Objects and Classes The program in Listing 4.5 shows many of the features that we discussed in this section: • Overloaded constructors • A call to another constructor with this(. . .) • A no-argument constructor • An object initialization block • A static initialization block • An instance field initialization Listing 4.5 ConstructorTest/ConstructorTest.java 1 import java.util.*; 2 3 4 5 6 7 8 9 10 11 12 13 /** * This program demonstrates object construction. * @version 1.02 2018-04-10 * @author Cay Horstmann */ public class ConstructorTest { public static void main(String[] args) { // fill the staff array with three Employee objects var staff = new Employee[3]; 14 staff[0] = new Employee("Harry", 40000); staff[1] = new Employee(60000); staff[2] = new Employee(); 15 16 17 18 // print out information about all Employee objects for (Employee e : staff) System.out.println("name=" + e.getName() + ",id=" + e.getId() + ",salary=" + e.getSalary()); 19 20 21 22 } 23 24 } 25 26 27 28 class Employee { private static int nextId; 29 30 31 32 private int id; private String name = ""; // instance field initialization private double salary; 33 From the Library of Hristo Dimov Hristov 4.6 Object Construction 179 // static initialization block static { var generator = new Random(); // set nextId to a random number between 0 and 9999 nextId = generator.nextInt(10000); } 34 35 36 37 38 39 40 41 // object initialization block { id = nextId; nextId++; } 42 43 44 45 46 47 // three overloaded constructors public Employee(String n, double s) { name = n; salary = s; } 48 49 50 51 52 53 54 public Employee(double s) { // calls the Employee(String, double) constructor this("Employee #" + nextId, s); } 55 56 57 58 59 60 // the default constructor public Employee() { // name initialized to ""--see above // salary not explicitly set--initialized to 0 // id initialized in initialization block } 61 62 63 64 65 66 67 68 public String getName() { return name; } 69 70 71 72 73 public double getSalary() { return salary; } 74 75 76 77 78 public int getId() { return id; } 79 80 81 82 83 } From the Library of Hristo Dimov Hristov 180 Chapter 4 Objects and Classes java.util.Random 1.0 • Random() constructs a new random number generator. • int nextInt(int n) 1.2 returns a random number between 0 and n – 1. 4.6.8 Object Destruction and the finalize Method Some object-oriented programming languages, notably C++, have explicit destructor methods for any cleanup code that may be needed when an object is no longer used. The most common activity in a destructor is reclaiming the memory set aside for objects. Since Java does automatic garbage collection, manual memory reclamation is not needed, so Java does not support destructors. Of course, some objects utilize a resource other than memory, such as a file or a handle to another object that uses system resources. In this case, it is important that the resource be reclaimed and recycled when it is no longer needed. If a resource needs to be closed as soon as you have finished using it, supply a close method that does the necessary cleanup. You can call the close method when you are done with the object. In Chapter 7, you will see how you can ensure that this method is called automatically. If you can wait until the virtual machine exits, add a “shutdown hook” with the method Runtime.addShutdownHook. As of Java 9, you can use the Cleaner class to register an action that is carried out when an object is no longer reachable (other than by the cleaner). These are uncommon situations in practice. See the API documentation for details on these two approaches. CAUTION: Do not use the finalize method for cleanup. That method was intended to be called before the garbage collector sweeps away an object. However, you simply cannot know when this method will be called, and it is now deprecated. 4.7 Packages Java allows you to group classes in a collection called a package. Packages are convenient for organizing your work and for separating your work from code From the Library of Hristo Dimov Hristov 4.7 Packages 181 libraries provided by others. In the following sections, you will learn how to use and create packages. 4.7.1 Package Names The main reason for using packages is to guarantee the uniqueness of class names. Suppose two programmers come up with the bright idea of supplying an Employee class. As long as both of them place their class into different packages, there is no conflict. In fact, to absolutely guarantee a unique package name, use an Internet domain name (which is known to be unique) written in reverse. You then use subpackages for different projects. For example, consider the domain horstmann.com. When written in reverse order, it turns into the package name com.horstmann. You can then append a project name, such as com.horstmann.corejava. If you then place the Employee class into that package, the “fully qualified” name becomes com.horstmann.corejava.Employee. NOTE: From the point of view of the compiler, there is absolutely no relationship between nested packages. For example, the packages java.util and java.util.jar have nothing to do with each other. Each is its own independent collection of classes. 4.7.2 Class Importation A class can use all classes from its own package and all public classes from other packages. You can access the public classes in another package in two ways. The first is simply to use the fully qualified name; that is, the package name followed by the class name. For example: java.time.LocalDate today = java.time.LocalDate.now(); That is obviously tedious. A simpler, and more common, approach is to use the import statement. The point of the import statement is to give you a shorthand to refer to the classes in the package. Once you add an import, you no longer have to give the classes their full names. You can import a specific class or the whole package. You place import statements at the top of your source files (but below any package statements). For example, you can import all classes in the java.time package with the statement import java.time.*; Then you can use LocalDate today = LocalDate.now(); From the Library of Hristo Dimov Hristov 182 Chapter 4 Objects and Classes without a package prefix. You can also import a specific class inside a package: import java.time.LocalDate; The java.time.* syntax is less tedious. It has no negative effect on code size. However, if you import classes explicitly, the reader of your code knows exactly which classes you use. TIP: In Eclipse, you can select the menu option Source → Organize Imports. Package statements such as import java.util.*; are automatically expanded into a list of specific imports such as import java.util.ArrayList; import java.util.Date; This is an extremely convenient feature. However, note that you can only use the * notation to import a single package. You cannot use import java.* or import java.*.* to import all packages with the java prefix. Most of the time, you just import the packages that you need, without worrying too much about them. The only time that you need to pay attention to packages is when you have a name conflict. For example, both the java.util and java.sql packages have a Date class. Suppose you write a program that imports both packages. import java.util.*; import java.sql.*; If you now use the Date class, you get a compile-time error: Date today; // ERROR--java.util.Date or java.sql.Date? The compiler cannot figure out which Date class you want. You can solve this problem by adding a specific import statement: import java.util.*; import java.sql.*; import java.util.Date; What if you really need both Date classes? Then use the full package name with every class name: var deadline = new java.util.Date(); var today = new java.sql.Date(. . .); Locating classes in packages is an activity of the compiler. The bytecodes in class files always use full package names to refer to other classes. From the Library of Hristo Dimov Hristov 4.7 Packages 183 C++ NOTE: C++ programmers sometimes confuse import with #include. The two have nothing in common. In C++, you must use #include to include the declarations of external features because the C++ compiler does not look inside any files except the one that it is compiling and its explicitly included header files. The Java compiler will happily look inside other files provided you tell it where to look. In Java, you can entirely avoid the import mechanism by explicitly naming all classes, such as java.util.Date. In C++, you cannot avoid the #include directives. The only benefit of the import statement is convenience. You can refer to a class by a name shorter than the full package name. For example, after an import java.util.* (or import java.util.Date) statement, you can refer to the java.util.Date class simply as Date. In C++, the construction analogous to the package mechanism is the namespace feature. Think of the package and import statements in Java as the analogs of the namespace and using directives in C++. 4.7.3 Static Imports A form of the import statement permits the importing of static methods and fields, not just classes. For example, if you add the directive import static java.lang.System.*; to the top of your source file, then you can use the static methods and fields of the System class without the class name prefix: out.println("Goodbye, World!"); // i.e., System.out exit(0); // i.e., System.exit You can also import a specific method or field: import static java.lang.System.out; In practice, it seems doubtful that many programmers will want to abbreviate System.out or System.exit. The resulting code seems less clear. On the other hand, sqrt(pow(x, 2) + pow(y, 2)) seems much clearer than Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2)) From the Library of Hristo Dimov Hristov 184 Chapter 4 Objects and Classes 4.7.4 Addition of a Class into a Package To place classes inside a package, put the name of the package at the top of your source file, before the code that defines the classes in the package. For example, the file Employee.java in Listing 4.7 starts out like this: package com.horstmann.corejava; public class Employee { . . . } If you don’t put a package statement in the source file, then the classes in that source file belong to the unnamed package. The unnamed package has no package name. Up to now, all our example classes were located in the unnamed package. Place source files into a subdirectory that matches the full package name. For example, all source files in the com.horstmann.corejava package should be in a subdirectory com/horstmann/corejava (com\horstmann\corejava on Windows). The compiler places the class files into the same directory structure. The program in Listings 4.6 and 4.7 is distributed over two packages: The PackageTest class belongs to the unnamed package, and the Employee class belongs to the com.horstmann.corejava package. Therefore, the Employee.java file must be in a subdirectory com/horstmann/corejava. In other words, the directory structure is as follows: . (base directory) ├─ PackageTest.java ├─ PackageTest.class └─ com/ └─ horstmann/ └─ corejava/ ├─ Employee.java └─ Employee.class To compile this program, simply change to the base directory and run the command javac PackageTest.java The compiler automatically finds the file com/horstmann/corejava/Employee.java and compiles it. Let’s look at a more realistic example, in which we don’t use the unnamed package but have classes distributed over several packages (com.horstmann.corejava and com.mycompany). From the Library of Hristo Dimov Hristov 4.7 Packages 185 . (base directory) └─ com/ ├─ horstmann/ │ └─ corejava/ │ ├─ Employee.java │ └─ Employee.class └─ mycompany/ ├─ PayrollApp.java └─ PayrollApp.class In this situation, you still must compile and run classes from the base directory—that is, the directory containing the com directory: javac com/mycompany/PayrollApp.java java com.mycompany.PayrollApp Note again that the compiler operates on files (with file separators and an extension .java), whereas the Java interpreter loads a class (with dot separators). TIP: Starting with the next chapter, we will use packages for the source code. That way, you can make an IDE project for each chapter instead of each section. CAUTION: The compiler does not check the directory structure when it compiles source files. For example, suppose you have a source file that starts with the directive package com.mycompany; You can compile the file even if it is not contained in a subdirectory com/mycompany. The source file will compile without errors if it doesn’t depend on other packages. However, the resulting program will not run unless you first move all class files to the right place. The virtual machine won’t find the classes if the packages don’t match the directories. Listing 4.6 PackageTest/PackageTest.java 1 2 import com.horstmann.corejava.*; // the Employee class is defined in that package 3 4 import static java.lang.System.*; 5 6 7 /** * This program demonstrates the use of packages. (Continues) From the Library of Hristo Dimov Hristov 186 Chapter 4 Objects and Classes Listing 4.6 (Continued) 8 9 10 11 12 13 14 15 16 17 * @version 1.11 2004-02-19 * @author Cay Horstmann */ public class PackageTest { public static void main(String[] args) { // because of the import statement, we don't have to use // com.horstmann.corejava.Employee here var harry = new Employee("Harry Hacker", 50000, 1989, 10, 1); 18 harry.raiseSalary(5); 19 20 // because of the static import statement, we don't have to use System.out here out.println("name=" + harry.getName() + ",salary=" + harry.getSalary()); 21 22 } 23 24 } Listing 4.7 PackageTest/com/horstmann/corejava/Employee.java 1 package com.horstmann.corejava; 2 3 // the classes in this file are part of this package 4 5 import java.time.*; 6 7 // import statements come after the package statement 8 9 10 11 12 13 14 15 16 17 /** * @version 1.11 2015-05-08 * @author Cay Horstmann */ public class Employee { private String name; private double salary; private LocalDate hireDay; 18 19 20 21 22 23 24 public Employee(String name, double salary, int year, int month, int day) { this.name = name; this.salary = salary; hireDay = LocalDate.of(year, month, day); } 25 From the Library of Hristo Dimov Hristov 4.7 Packages 187 public String getName() { return name; } 26 27 28 29 30 public double getSalary() { return salary; } 31 32 33 34 35 public LocalDate getHireDay() { return hireDay; } 36 37 38 39 40 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 41 42 43 44 45 46 } 4.7.5 Package Access You have already encountered the access modifiers public and private. Features tagged as public can be used by any class. Private features can be used only by the class that defines them. If you don’t specify either public or private, the feature (that is, the class, method, or variable) can be accessed by all methods in the same package. Consider the program in Listing 4.2. The Employee class was not defined as a public class. Therefore, only the other classes (such as EmployeeTest) in the same package—the unnamed package in this case—can access it. For classes, this is a reasonable default. However, for variables, this was an unfortunate choice. Variables must explicitly be marked private, or they will default to having package access. This, of course, breaks encapsulation. The problem is that it is awfully easy to forget to type the private keyword. Here is an example from the Window class in the java.awt package, which is part of the source code supplied with the JDK: public class Window extends Container { String warningString; . . . } Note that the warningString variable is not private! That means the methods of all classes in the java.awt package can access this variable and set it to whatever From the Library of Hristo Dimov Hristov 188 Chapter 4 Objects and Classes they like (such as "Trust me!"). Actually, the only methods that access this variable are in the Window class, so it would have been entirely appropriate to make the variable private. Perhaps the programmer typed the code in a hurry and simply forgot the private modifier? Perhaps nobody cared? After more than twenty years, that variable is still not private. Not only that—new fields have been added to the class over time, and about half of them aren’t private either. This can be a problem. By default, packages are not closed entities. That is, anyone can add more classes to a package. Of course, hostile or clueless programmers can then add code that modifies variables with package access. For example, in early versions of Java, it was an easy matter to smuggle another class into the java.awt package. Simply start out the class with package java.awt; Then, place the resulting class file inside a subdirectory java/awt somewhere on the class path, and you have gained access to the internals of the java.awt package. Through this subterfuge, it was possible to set the warning string (see Figure 4.9). Figure 4.9 Changing the warning string in an applet window Starting with version 1.2, the JDK implementors rigged the class loader to explicitly disallow loading of user-defined classes whose package name starts with "java.". Of course, your own classes don’t benefit from that protection. Another mechanism, now obsolete, lets a JAR file declare packages as sealed, preventing third parties from augmenting them. Nowadays, you should use modules to encapsulate packages. We discuss modules in detail in Chapter 9 of Volume II. From the Library of Hristo Dimov Hristov 4.7 Packages 189 4.7.6 The Class Path As you have seen, classes are stored in subdirectories of the file system. The path to the class must match the package name. Class files can also be stored in a JAR (Java archive) file. A JAR file contains multiple class files and subdirectories in a compressed format, saving space and improving performance. When you use a third-party library in your programs, you will usually be given one or more JAR files to include. You will see in Chapter 11 how to create your own JAR files. TIP: JAR files use the ZIP format to organize files and subdirectories. You can use any ZIP utility to peek inside JAR files. To share classes among programs, you need to do the following: 1. 2. 3. Place your class files inside a directory—for example, /home/user/classdir. Note that this directory is the base directory for the package tree. If you add the class com.horstmann.corejava.Employee, then the Employee.class file must be located in the subdirectory /home/user/classdir/com/horstmann/corejava. Place any JAR files inside a directory—for example, /home/user/archives. Set the class path. The class path is the collection of all locations that can contain class files. In UNIX, the elements on the class path are separated by colons: /home/user/classdir:.:/home/user/archives/archive.jar In Windows, they are separated by semicolons: c:\classdir;.;c:\archives\archive.jar In both cases, the period denotes the current directory. This class path contains • The base directory /home/user/classdir or c:\classdir; • The current directory (.); and • The JAR file /home/user/archives/archive.jar or c:\archives\archive.jar. Starting with Java 6, you can specify a wildcard for a JAR file directory, like this: /home/user/classdir:.:/home/user/archives/'*' From the Library of Hristo Dimov Hristov 190 Chapter 4 Objects and Classes or c:\classdir;.;c:\archives\* In UNIX, the * must be escaped to prevent shell expansion. All JAR files (but not .class files) in the archives directory are included in this class path. The Java API is always searched for classes; don’t include it explicitly in the class path. CAUTION: The javac compiler always looks for files in the current directory, but the java virtual machine launcher only looks into the current directory if the “.” directory is on the class path. If you have no class path set, it’s not a problem— the default class path consists of the “.” directory. But if you have set the class path and forgot to include the “.” directory, your programs will compile without error, but they won’t run. The class path lists all directories and archive files that are starting points for locating classes. Let’s consider our sample class path: /home/user/classdir:.:/home/user/archives/archive.jar Suppose the virtual machine searches for the class file of the com.horstmann .corejava.Employee class. It first looks in the Java API classes. It won’t find the class file there, so it turns to the class path. It then looks for the following files: • /home/user/classdir/com/horstmann/corejava/Employee.class • com/horstmann/corejava/Employee.class starting from the current directory • com/horstmann/corejava/Employee.class inside /home/user/archives/archive.jar The compiler has a harder time locating files than does the virtual machine. If you refer to a class without specifying its package, the compiler first needs to find out the package that contains the class. It consults all import directives as possible sources for the class. For example, suppose the source file contains directives import java.util.*; import com.horstmann.corejava.*; and the source code refers to a class Employee. The compiler then tries to find java.lang.Employee (because the java.lang package is always imported by default), java.util.Employee, com.horstmann.corejava.Employee, and Employee in the current package. It searches for each of these classes in all of the locations of the class path. It From the Library of Hristo Dimov Hristov 4.7 Packages 191 is a compile-time error if more than one class is found. (Fully qualified class names must be unique, so the order of the import statements doesn’t matter.) The compiler goes one step further. It looks at the source files to see if the source is newer than the class file. If so, the source file is recompiled automatically. Recall that you can import only public classes from other packages. A source file can only contain one public class, and the names of the file and the public class must match. Therefore, the compiler can easily locate source files for public classes. However, you can import nonpublic classes from the current package. These classes may be defined in source files with different names. If you import a class from the current package, the compiler searches all source files of the current package to see which one defines the class. 4.7.7 Setting the Class Path It is best to specify the class path with the option -classpath (or -cp or, as of Java 9, --class-path): java -classpath /home/user/classdir:.:/home/user/archives/archive.jar MyProg or java -classpath c:\classdir;.;c:\archives\archive.jar MyProg The entire command must be typed onto a single line. It is a good idea to place such a long command line into a shell script or a batch file. Using the -classpath option is the preferred approach for setting the class path. An alternate approach is the CLASSPATH environment variable. The details depend on your shell. With the Bourne Again shell (bash), use the command export CLASSPATH=/home/user/classdir:.:/home/user/archives/archive.jar With the Windows shell, use set CLASSPATH=c:\classdir;.;c:\archives\archive.jar The class path is set until the shell exits. CAUTION: Some people recommend to set the CLASSPATH environment variable permanently. This is generally a bad idea. People forget the global setting, and are surprised when their classes are not loaded properly. A particularly reprehensible example is Apple’s QuickTime installer in Windows. For several years, it globally set CLASSPATH to point to a JAR file it needed, but did not include the current directory in the classpath. As a result, countless Java programmers were driven to distraction when their programs compiled but failed to run. From the Library of Hristo Dimov Hristov 192 Chapter 4 Objects and Classes CAUTION: In the past, some people recommended to bypass the class path altogether, by dropping all JAR files into the jre/lib/ext directory. That mechanism is obsolete with Java 9, but it was always bad advice. It was easy to get confused when long-forgotten classes were loaded from the extension directory. NOTE: As of Java 9, classes can also be loaded from the module path. We discuss modules and the module path in Chapter 9 of Volume II. 4.8 JAR Files When you package your application, you want to give your users a single file, not a directory structure filled with class files. Java Archive (JAR) files were designed for this purpose. A JAR file can contain both class files and other file types such as image and sound files. Moreover, JAR files are compressed, using the familiar ZIP compression format. 4.8.1 Creating JAR files Use the jar tool to make JAR files. (In the default JDK installation, it’s in the jdk/bin directory.) The most common command to make a new JAR file uses the following syntax: jar cvf jarFileName file1 file2 . . . For example: jar cvf CalculatorClasses.jar *.class icon.gif In general, the jar command has the following format: jar options file1 file2 . . . Table 4.2 lists all the options for the jar program. They are similar to the options of the UNIX tar command. You can package application programs and code libraries into JAR files. For example, if you want to send mail in a Java program, you use a library that is packaged in a file javax.mail.jar. From the Library of Hristo Dimov Hristov 4.8 JAR Files 193 Table 4.2 jar Program Options Option Description c Creates a new or empty archive and adds files to it. If any of the specified file names are directories, the jar program processes them recursively. C Temporarily changes the directory. For example, jar cvf jarFileName.jar -C classes *.class changes to the classes subdirectory to add class files. e Creates an entry point in the manifest (see Section 4.8.3). f Specifies the JAR file name as the second command-line argument. If this parameter is missing, jar will write the result to standard output (when creating a JAR file) or read it from standard input (when extracting or tabulating a JAR file). i Creates an index file (for speeding up lookups in a large archive). m Adds a manifest to the JAR file. A manifest is a description of the archive contents and origin. Every archive has a default manifest, but you can supply your own if you want to authenticate the contents of the archive. M Does not create a manifest file for the entries. t Displays the table of contents. u Updates an existing JAR file. v Generates verbose output. x Extracts files. If you supply one or more file names, only those files are extracted. Otherwise, all files are extracted. 0 Stores without ZIP compression. 4.8.2 The Manifest In addition to class files, images, and other resources, each JAR file contains a manifest file that describes special features of the archive. The manifest file is called MANIFEST.MF and is located in a special META-INF subdirectory of the JAR file. The minimum legal manifest is quite boring—just Manifest-Version: 1.0 From the Library of Hristo Dimov Hristov 194 Chapter 4 Objects and Classes Complex manifests can have many more entries. The manifest entries are grouped into sections. The first section in the manifest is called the main section. It applies to the whole JAR file. Subsequent entries can specify properties of named entities such as individual files, packages, or URLs. Those entries must begin with a Name entry. Sections are separated by blank lines. For example: Manifest-Version: 1.0 lines describing this archive Name: Woozle.class lines describing this file Name: com/mycompany/mypkg/ lines describing this package To edit the manifest, place the lines that you want to add to the manifest into a text file. Then run jar cfm jarFileName manifestFileName . . . For example, to make a new JAR file with a manifest, run jar cfm MyArchive.jar manifest.mf com/mycompany/mypkg/*.class To update the manifest of an existing JAR file, place the additions into a text file and use a command such as jar ufm MyArchive.jar manifest-additions.mf NOTE: See https://docs.oracle.com/javase/10/docs/specs/jar/jar.html for more information on the JAR and manifest file formats. 4.8.3 Executable JAR Files You can use the e option of the jar command to specify the entry point of your program—the class that you would normally specify when invoking the java program launcher: jar cvfe MyProgram.jar com.mycompany.mypkg.MainAppClass files to add Alternatively, you can specify the main class of your program in the manifest, including a statement of the form Main-Class: com.mycompany.mypkg.MainAppClass Do not add a .class extension to the main class name. From the Library of Hristo Dimov Hristov 4.8 JAR Files 195 CAUTION: The last line in the manifest must end with a newline character. Otherwise, the manifest will not be read correctly. It is a common error to produce a text file containing just the Main-Class line without a line terminator. With either method, users can simply start the program as java -jar MyProgram.jar Depending on the operating system configuration, users may even be able to launch the application by double-clicking the JAR file icon. Here are behaviors for various operating systems: • On Windows, the Java runtime installer creates a file association for the “.jar” extension that launches the file with the javaw -jar command. (Unlike the java command, the javaw command doesn’t open a shell window.) • On Mac OS X, the operating system recognizes the “.jar” file extension and executes the Java program when you double-click a JAR file. However, a Java program in a JAR file does not have the same feel as a native application. On Windows, you can use third-party wrapper utilities that turn JAR files into Windows executables. A wrapper is a Windows program with the familiar .exe extension that locates and launches the Java virtual machine (JVM) or tells the user what to do when no JVM is found. There are a number of commercial and open source products, such as Launch4J (http://launch4j .sourceforge.net) and IzPack (http://izpack.org). 4.8.4 Multi-Release JAR Files With the introduction of modules and strong encapsulation of packages, some previously accessible internal APIs are no longer available. For example, JavaFX 8 had an internal class com.sun.javafx.css.CssParser. If you used it to parse a style sheet, then you will find that your program no longer compiles. The remedy is simple—switch to javafx.css.CssParser, which is available in Java 9. But now you have a problem. You need to distribute different applications for Java 8 and Java 9 users, or you need to play tricks with class loading and reflection. To solve problems such as this one, Java 9 introduces multi-release JARs that can contain class files for different Java releases. For backwards compatibility, the additional class files are placed in the META-INF/versions directory: From the Library of Hristo Dimov Hristov 196 Chapter 4 Objects and Classes Application.class BuildingBlocks.class Util.class META-INF ├─ MANIFEST.MF (with line Multi-Release: true) └─ versions ├─ 9 │ ├─ Application.class │ └─ BuildingBlocks.class └─ 10 └─ BuildingBlocks.class Suppose the Application class makes use of the CssParser class. Then the legacy Application.class file can be compiled to use com.sun.javafx.css.CssParser, while the Java 9 version uses javafx.css.CssParser. Java 8 knows nothing about the META-INF/versions directory and will simply load the legacy classes. When the JAR file is read by Java 9, the new version is used instead. To add versioned class files, use the --release flag: jar uf MyProgram.jar --release 9 Application.class To build a multi-release JAR file from scratch, use the -C option and switch to a different class file directory for each version: jar cf MyProgram.jar -C bin/8 . --release 9 -C bin/9 Application.class When compiling for different releases, use the --release flag and the -d flag to specify the output directory: javac -d bin/8 --release 8 . . . As of Java 9, the -d option creates the directory if it doesn’t exist. The --release flag is also new with Java 9. In older versions, you needed to use the -source, -target, and -bootclasspath flags. The JDK now ships with symbol files for two prior versions of the API. In Java 9, you can compile with --release set to 9, 8, or 7. Multi-release JARs are not intended for different versions of a program or library. The public API of all classes should be the same for both releases. The sole purpose of multi-release JARs is to enable a particular version of your program or library to work with multiple JDK releases. If you add functionality or change an API, you should provide a new version of the JAR instead. From the Library of Hristo Dimov Hristov 4.8 JAR Files 197 NOTE: Tools such as javap are not retrofitted to handle multi-release JAR files. If you call javap -classpath MyProgram.jar Application.class you get the base version of the class (which, after all, is supposed to have the same public API as the newer version). If you must look at the newer version, call javap -classpath MyProgram.jar\!/META-INF/versions/9/Application.class 4.8.5 A Note about Command-Line Options The options of commands in the Java Development Kit have traditionally used single dashes followed by multiletter option names, such as java -jar . . . javac -Xlint:unchecked -classpath . . . The exception was the jar command, which followed the classic option format of the tar command without dashes: jar cvf . . . Starting with Java 9, the Java tools are moving towards a more common option format where multiletter option names are preceded by double dashes, with single-letter shortcuts for common options. For example, the Linux ls command can be called with a “human-readable” option as ls --human-readable or ls -h As of Java 9, you can use --version instead of -version and --class-path instead of -classpath. As you will see in Chapter 9 of Volume II, the --module-path option has a shortcut -p. You can find the details in the JEP 293 enhancement request at http://openjdk .java.net/jeps/293. As part of this cleanup, the authors also propose to standardize option arguments. Arguments of options with -- and multiple letters are separated by whitespace or an = sign: javac --class-path /home/user/classdir . . . or javac --class-path=/home/user/classdir . . . From the Library of Hristo Dimov Hristov 198 Chapter 4 Objects and Classes Arguments of single-letter options can be separated by whitespace or directly follow the option: javac -p moduledir . . . or javac -pmoduledir . . . CAUTION: The latter doesn’t currently work, and it also seems like a bad idea in general. Why invite conflicts with legacy options if the module directory happens to be arameters or rocessor? Single-letter options without arguments can be grouped together: jar -cvf MyProgram.jar -e mypackage.MyProgram */*.class CAUTION: That doesn’t currently work, and it is bound to lead to confusion. Suppose javac gains a -c option. Does javac -cp mean javac -c -p, or does the legacy -cp take precedence? This has created a muddle that will hopefully get cleaned up over time. As much as we’d like to move away from the archaic jar options, it seems best to wait until the dust has settled. But if you want to be thoroughly modern, you can safely use the long options of the jar command: jar --create --verbose --file jarFileName file1 file2 . . . Single-letter options also work if you don’t group them: jar -c -v -f jarFileName file1 file2 . . . 4.9 Documentation Comments The JDK contains a very useful tool, called javadoc, that generates HTML documentation from your source files. In fact, the online API documentation that we described in Chapter 3 is simply the result of running javadoc on the source code of the standard Java library. If you add comments that start with the special delimiter /** to your source code, you too can easily produce professional-looking documentation. This is a very nice approach because it lets you keep your code and documentation in one place. If you put your documentation into a separate file, then, as you probably know, the code and comments tend to diverge over time. When From the Library of Hristo Dimov Hristov 4.9 Documentation Comments 199 documentation comments are in the same file as the source code, it is an easy matter to update both and run javadoc again. 4.9.1 Comment Insertion The javadoc utility extracts information for the following items: • • • • • Modules Packages Public classes and interfaces Public and protected fields Public and protected constructors and methods Protected features are introduced in Chapter 5, interfaces in Chapter 6, and modules in Chapter 9 of Volume II. You can (and should) supply a comment for each of these features. Each comment is placed immediately above the feature it describes. A comment starts with a /** and ends with a */. Each /** . . . */ documentation comment contains free-form text followed by tags. A tag starts with an @, such as @since or @param. The first sentence of the free-form text should be a summary statement. The utility automatically generates summary pages that extract these sentences. javadoc In the free-form text, you can use HTML modifiers such as . . . for emphasis, . . . for strong emphasis, <ul>/<li> for bulleted lists, and <img . . ./> to include an image. To type monospaced code, use {@code . . . } instead of <code>. . .</code>—then you don’t have to worry about escaping < characters inside the code. NOTE: If your comments contain links to other files such as images (for example, diagrams or images of user interface components), place those files into a subdirectory, named doc-files, of the directory containing the source file. The javadoc utility will copy the doc-files directories and their contents from the source directory to the documentation directory. You need to use the doc-files directory in your link, for example <img src="doc-files/uml.png" alt="UML diagram"/>. 4.9.2 Class Comments The class comment must be placed after any import statements, directly before the class definition. From the Library of Hristo Dimov Hristov 200 Chapter 4 Objects and Classes Here is an example of a class comment: /** * A {@code Card} object represents a playing card, such * as "Queen of Hearts". A card has a suit (Diamond, Heart, * Spade or Club) and a value (1 = Ace, 2 . . . 10, 11 = Jack, * 12 = Queen, 13 = King) */ public class Card { . . . } NOTE: There is no need to add an * in front of every line. For example, the following comment is equally valid: /** A <code>Card</code> object represents a playing card, such as "Queen of Hearts". A card has a suit (Diamond, Heart, Spade or Club) and a value (1 = Ace, 2 . . . 10, 11 = Jack, 12 = Queen, 13 = King). */ However, most IDEs supply the asterisks automatically and rearrange them when the line breaks change. 4.9.3 Method Comments Each method comment must immediately precede the method that it describes. In addition to the general-purpose tags, you can use the following tags: • @param variable description This tag adds an entry to the “parameters” section of the current method. The description can span multiple lines and can use HTML tags. All @param tags for one method must be kept together. • @return description This tag adds a “returns” section to the current method. The description can span multiple lines and can use HTML tags. • @throws class description This tag adds a note that this method may throw an exception. Exceptions are the topic of Chapter 7. Here is an example of a method comment: From the Library of Hristo Dimov Hristov 4.9 Documentation Comments 201 /** * Raises the salary of an employee. * @param byPercent the percentage by which to raise the salary (e.g., 10 means 10%) * @return the amount of the raise */ public double raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; return raise; } 4.9.4 Field Comments You only need to document public fields—generally that means static constants. For example: /** * The "Hearts" card suit */ public static final int HEARTS = 1; 4.9.5 General Comments The tag @since text makes a “since” entry. The text can be any description of the version that introduced this feature. For example, @since 1.7.1. The following tags can be used in class documentation comments: • @author name This tag makes an “author” entry. You can have multiple @author tags, one for each author. Don’t feel compelled to use this tag—your version control system does a more thorough job tracking authorship. • @version text This tag makes a “version” entry. The text can be any description of the current version. You can use hyperlinks to other relevant parts of the javadoc documentation, or to external documents, with the @see and @link tags. The tag @see reference adds a hyperlink in the “see also” section. It can be used with both classes and methods. Here, reference can be one of the following: package.class#feature label <a href=". . .">label</a> "text" From the Library of Hristo Dimov Hristov 202 Chapter 4 Objects and Classes The first case is the most useful. You supply the name of a class, method, or variable, and javadoc inserts a hyperlink to the documentation. For example, @see com.horstmann.corejava.Employee#raiseSalary(double) makes a link to the raiseSalary(double) method in the com.horstmann.corejava.Employee class. You can omit the name of the package, or both the package and class names. Then, the feature will be located in the current package or class. Note that you must use a #, not a period, to separate the class from the method or variable name. The Java compiler itself is highly skilled in determining the various meanings of the period character as separator between packages, subpackages, classes, inner classes, and methods and variables. But the javadoc utility isn’t quite as clever, so you have to help it along. If the @see tag is followed by a < character, then you need to specify a hyperlink. You can link to any URL you like. For example: @see <a href="www.horstmann.com/corejava.html">The Core Java home page</a> In each of these cases, you can specify an optional label that will appear as the link anchor. If you omit the label, the user will see the target code name or URL as the anchor. If the @see tag is followed by a " character, then the text is displayed in the “see also” section. For example: @see "Core Java 2 volume 2" You can add multiple @see tags for one feature, but you must keep them all together. If you like, you can place hyperlinks to other classes or methods anywhere in any of your documentation comments. Insert a special tag of the form {@link package.class#feature label} anywhere in a comment. The feature description follows the same rules as for the @see tag. Finally, as of Java 9, you can use the {@index entry} tag to add an entry to the search box. 4.9.6 Package Comments Place the class, method, and variable comments directly into the Java source files, delimited by /** . . . */ documentation comments. However, to generate package comments, you need to add a separate file in each package directory. You have two choices: From the Library of Hristo Dimov Hristov 4.9 1. 2. Documentation Comments 203 Supply a Java file named package-info.java. The file must contain an initial Javadoc comment, delimited with /** and */, followed by a package statement. It should contain no further code or comments. Supply an HTML file named package.html. All text between the tags <body>. . .</body> is extracted. 4.9.7 Comment Extraction Here, docDirectory is the name of the directory where you want the HTML files to go. Follow these steps: 1. 2. Change to the directory that contains the source files you want to document. If you have nested packages to document, such as com.horstmann .corejava, you must be working in the directory that contains the subdirectory com. (This is the directory that contains the overview.html file, if you supplied one.) Run the command javadoc -d docDirectory nameOfPackage for a single package. Or, run javadoc -d docDirectory nameOfPackage1 nameOfPackage2. . . to document multiple packages. If your files are in the unnamed package, run instead javadoc -d docDirectory *.java If you omit the -d docDirectory option, the HTML files are extracted to the current directory. That can get messy, and we don’t recommend it. The javadoc program can be fine-tuned by numerous command-line options. For example, you can use the -author and -version options to include the @author and @version tags in the documentation. (By default, they are omitted.) Another useful option is -link, to include hyperlinks to standard classes. For example, if you use the command javadoc -link http://docs.oracle.com/javase/9/docs/api *.java all standard library classes are automatically linked to the documentation on the Oracle web site. If you use the -linksource option, each source file is converted to HTML (without color coding, but with line numbers), and each class and method name turns into a hyperlink to the source. From the Library of Hristo Dimov Hristov 204 Chapter 4 Objects and Classes You can also supply an overview comment for all source files. Place it in a file such as overview.html and run the javadoc tool with the command line option -overview filename. All text between the tags <body>. . .</body> is extracted. The content is displayed when the user selects “Overview” from the navigation bar. For additional options, we refer you to the online documentation of the javadoc utility at https://docs.oracle.com/javase/9/javadoc/javadoc.htm. 4.10 Class Design Hints Without trying to be comprehensive or tedious, we want to end this chapter with some hints that will make your classes more acceptable in well-mannered OOP circles. 1. Always keep data private. This is first and foremost; doing anything else violates encapsulation. You may need to write an accessor or mutator method occasionally, but you are still better off keeping the instance fields private. Bitter experience shows that the data representation may change, but how this data are used will change much less frequently. When data are kept private, changes in their representation will not affect the users of the class, and bugs are easier to detect. 2. Always initialize data. Java won’t initialize local variables for you, but it will initialize instance fields of objects. Don’t rely on the defaults, but initialize all variables explicitly, either by supplying a default or by setting defaults in all constructors. 3. Don’t use too many basic types in a class. The idea is to replace multiple related uses of basic types with other classes. This keeps your classes easier to understand and to change. For example, replace the following instance fields in a Customer class: private private private private String street; String city; String state; int zip; with a new class called Address. This way, you can easily cope with changes to addresses, such as the need to deal with international addresses. From the Library of Hristo Dimov Hristov 4.10 4. Class Design Hints 205 Not all fields need individual field accessors and mutators. You may need to get and set an employee’s salary. You certainly won’t need to change the hiring date once the object is constructed. And, quite often, objects have instance fields that you don’t want others to get or set, such as an array of state abbreviations in an Address class. 5. Break up classes that have too many responsibilities. This hint is, of course, vague: “too many” is obviously in the eye of the beholder. However, if there is an obvious way to break one complicated class into two classes that are conceptually simpler, seize the opportunity. (On the other hand, don’t go overboard; ten classes, each with only one method, are usually an overkill.) Here is an example of a bad design: public class CardDeck // bad design { private int[] value; private int[] suit; public public public public public CardDeck() { . . . } void shuffle() { . . . } int getTopValue() { . . . } int getTopSuit() { . . . } void draw() { . . . } } This class really implements two separate concepts: a deck of cards, with its shuffle and draw methods, and a card, with the methods to inspect its value and suit. It makes sense to introduce a Card class that represents an individual card. Now you have two classes, each with its own responsibilities: public class CardDeck { private Card[] cards; public public public public CardDeck() { . . . } void shuffle() { . . . } Card getTop() { . . . } void draw() { . . . } } public class Card { private int value; private int suit; From the Library of Hristo Dimov Hristov 206 Chapter 4 Objects and Classes public Card(int aValue, int aSuit) { . . . } public int getValue() { . . . } public int getSuit() { . . . } } 6. Make the names of your classes and methods reflect their responsibilities. Just as variables should have meaningful names that reflect what they represent, so should classes. (The standard library certainly contains some dubious examples, such as the Date class that describes time.) A good convention is that a class name should be a noun (Order), or a noun preceded by an adjective (RushOrder) or a gerund (an “-ing” word, as in BillingAddress). As for methods, follow the standard convention that accessor methods begin with a lowercase get (getSalary) and mutator methods use a lowercase set (setSalary). 7. Prefer immutable classes. The LocalDate class, and other classes from the java.time package, are immutable—no method can modify the state of an object. Instead of mutating objects, methods such as plusDays return new objects with the modified state. The problem with mutation is that it can happen concurrently when multiple threads try to update an object at the same time. The results are unpredictable. When classes are immutable, it is safe to share their objects among multiple threads. Therefore, it is a good idea to make classes immutable when you can. This is particularly easy with classes that represent values, such as a string or a point in time. Computations can simply yield new values instead of updating existing ones. Of course, not all classes should be immutable. It would be strange to have the raiseSalary method return a new Employee object when an employee gets a raise. In this chapter, we covered the fundamentals of objects and classes that make Java an “object-based” language. In order to be truly object-oriented, a programming language must also support inheritance and polymorphism. The Java support for these features is the topic of the next chapter. From the Library of Hristo Dimov Hristov CHAPTER 5 Inheritance In this chapter • 5.1 • 5.2 Object: The Cosmic Superclass, page 232 Classes, Superclasses, and Subclasses, page 208 • 5.3 Generic Array Lists, page 248 • 5.4 Object Wrappers and Autoboxing, page 256 • 5.5 Methods with a Variable Number of Parameters, page 260 • 5.6 Enumeration Classes, page 261 • 5.7 Reflection, page 264 • 5.8 Design Hints for Inheritance, page 290 Chapter 4 introduced you to classes and objects. In this chapter, you will learn about inheritance, another fundamental concept of object-oriented programming. The idea behind inheritance is that you can create new classes that are built on existing classes. When you inherit from an existing class, you reuse (or inherit) its methods, and you can add new methods and fields to adapt your new class to new situations. This technique is essential in Java programming. This chapter also covers reflection, the ability to find out more about classes and their properties in a running program. Reflection is a powerful feature, but it is undeniably complex. Since reflection is of greater interest to tool builders than to application programmers, you can probably glance over that part of the chapter upon first reading and come back to it later. 207 From the Library of Hristo Dimov Hristov 208 Chapter 5 Inheritance 5.1 Classes, Superclasses, and Subclasses Let’s return to the Employee class that we discussed in the previous chapter. Suppose (alas) you work for a company where managers are treated differently from other employees. Managers are, of course, just like employees in many respects. Both employees and managers are paid a salary. However, while employees are expected to complete their assigned tasks in return for receiving their salary, managers get bonuses if they actually achieve what they are supposed to do. This is the kind of situation that cries out for inheritance. Why? Well, you need to define a new class, Manager, and add functionality. But you can retain some of what you have already programmed in the Employee class, and all the fields of the original class can be preserved. More abstractly, there is an obvious “is–a” relationship between Manager and Employee. Every manager is an employee: This “is–a” relationship is the hallmark of inheritance. NOTE: In this chapter, we use the classic example of employees and managers, but we must ask you to take this example with a grain of salt. In the real world, an employee can become a manager, so you would want to model being a manager as a role of an employee, not a subclass. In our example, however, we assume the corporate world is populated by two kinds of people: those who are forever employees, and those who have always been managers. 5.1.1 Defining Subclasses Here is how you define a Manager class that inherits from the Employee class. Use the Java keyword extends to denote inheritance. public class Manager extends Employee { added methods and fields } C++ NOTE: Inheritance is similar in Java and C++. Java uses the extends keyword instead of the : token. All inheritance in Java is public inheritance; there is no analog to the C++ features of private and protected inheritance. The keyword extends indicates that you are making a new class that derives from an existing class. The existing class is called the superclass, base class, or parent class. The new class is called the subclass, derived class, or child class. The terms superclass and subclass are those most commonly used by Java programmers, although some programmers prefer the parent/child analogy, which also ties in nicely with the “inheritance” theme. From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 209 The Employee class is a superclass, but not because it is superior to its subclass or contains more functionality. In fact, the opposite is true: Subclasses have more functionality than their superclasses. For example, as you will see when we go over the rest of the Manager class code, the Manager class encapsulates more data and has more functionality than its superclass Employee. NOTE: The prefixes super and sub come from the language of sets used in theoretical computer science and mathematics. The set of all employees contains the set of all managers, and thus is said to be a superset of the set of managers. Or, to put it another way, the set of all managers is a subset of the set of all employees. Our Manager class has a new field to store the bonus, and a new method to set it: public class Manager extends Employee { private double bonus; . . . public void setBonus(double bonus) { this.bonus = bonus; } } There is nothing special about these methods and fields. If you have a Manager object, you can simply apply the setBonus method. Manager boss = . . .; boss.setBonus(5000); Of course, if you have an Employee object, you cannot apply the setBonus method—it is not among the methods defined in the Employee class. However, you can use methods such as getName and getHireDay with Manager objects. Even though these methods are not explicitly defined in the Manager class, they are automatically inherited from the Employee superclass. Similarly, the fields name, salary, and hireDay are taken from the superclass. Every object has four fields: name, salary, hireDay, and bonus. Manager When defining a subclass by extending its superclass, you only need to indicate the differences between the subclass and the superclass. When designing classes, you place the most general methods in the superclass and more specialized methods in its subclasses. Factoring out common functionality by moving it to a superclass is routine in object-oriented programming. From the Library of Hristo Dimov Hristov 210 Chapter 5 Inheritance 5.1.2 Overriding Methods Some of the superclass methods are not appropriate for the Manager subclass. In particular, the getSalary method should return the sum of the base salary and the bonus. You need to supply a new method to override the superclass method: public class Manager extends Employee { . . . public double getSalary() { . . . } . . . } How can you implement this method? At first glance, it appears to be simple—just return the sum of the salary and bonus fields: public double getSalary() { return salary + bonus; // won't work } However, that won’t work. Recall that only the Employee methods have direct access to the private fields of the Employee class. This means that the getSalary method of the Manager class cannot directly access the salary field. If the Manager methods want to access those private fields, they have to do what every other method does—use the public interface, in this case the public getSalary method of the Employee class. So, let’s try again. You need to call getSalary instead of simply accessing the salary field: public double getSalary() { double baseSalary = getSalary(); // still won't work return baseSalary + bonus; } Now, the problem is that the call to getSalary simply calls itself, because the Manager class has a getSalary method (namely, the method we are trying to implement). The consequence is an infinite chain of calls to the same method, leading to a program crash. We need to indicate that we want to call the getSalary method of the Employee superclass, not the current class. You use the special keyword super for this purpose. The call From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 211 super.getSalary() calls the getSalary method of the Employee class. Here is the correct version of the getSalary method for the Manager class: public double getSalary() { double baseSalary = super.getSalary(); return baseSalary + bonus; } NOTE: Some people think of super as being analogous to the this reference. However, that analogy is not quite accurate: super is not a reference to an object. For example, you cannot assign the value super to another object variable. Instead, super is a special keyword that directs the compiler to invoke the superclass method. As you saw, a subclass can add fields, and it can add methods or override the methods of the superclass. However, inheritance can never take away any fields or methods. C++ NOTE: Java uses the keyword super to call a superclass method. In C++, you would use the name of the superclass with the :: operator instead. For example, the getSalary method of the Manager class would call Employee::getSalary instead of super.getSalary. 5.1.3 Subclass Constructors To complete our example, let us supply a constructor. public Manager(String name, double salary, int year, int month, int day) { super(name, salary, year, month, day); bonus = 0; } Here, the keyword super has a different meaning. The instruction super(name, salary, year, month, day); is shorthand for “call the constructor of the Employee superclass with n, s, year, month, and day as parameters.” Since the Manager constructor cannot access the private fields of the Employee class, it must initialize them through a constructor. The constructor is invoked From the Library of Hristo Dimov Hristov 212 Chapter 5 Inheritance with the special super syntax. The call using super must be the first statement in the constructor for the subclass. If the subclass constructor does not call a superclass constructor explicitly, the no-argument constructor of the superclass is invoked. If the superclass does not have a no-argument constructor and the subclass constructor does not call another superclass constructor explicitly, the Java compiler reports an error. NOTE: Recall that the this keyword has two meanings: to denote a reference to the implicit parameter and to call another constructor of the same class. Likewise, the super keyword has two meanings: to invoke a superclass method and to invoke a superclass constructor. When used to invoke constructors, the this and super keywords are closely related. The constructor calls can only occur as the first statement in another constructor. The constructor parameters are either passed to another constructor of the same class (this) or a constructor of the superclass (super). C++ NOTE: In a C++ constructor, you do not call super, but you use the initializer list syntax to construct the superclass. The Manager constructor would look like this in C++: // C++ Manager::Manager(String name, double salary, int year, int month, int day) : Employee(name, salary, year, month, day) { bonus = 0; } After you redefine the getSalary method for Manager objects, managers will automatically have the bonus added to their salaries. Here’s an example of this at work. We make a new manager and set the manager’s bonus: Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15); boss.setBonus(5000); We make an array of three employees: var staff = new Employee[3]; We populate the array with a mix of managers and employees: From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 213 staff[0] = boss; staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1); staff[2] = new Employee("Tony Tester", 40000, 1990, 3, 15); We print out everyone’s salary: for (Employee e : staff) System.out.println(e.getName() + " " + e.getSalary()); This loop prints the following data: Carl Cracker 85000.0 Harry Hacker 50000.0 Tommy Tester 40000.0 Now staff[1] and staff[2] each print their base salary because they are Employee objects. However, staff[0] is a Manager object whose getSalary method adds the bonus to the base salary. What is remarkable is that the call e.getSalary() picks out the correct getSalary method. Note that the declared type of e is Employee, but the actual type of the object to which e refers can be either Employee or Manager. When e refers to an Employee object, the call e.getSalary() calls the getSalary method of the Employee class. However, when e refers to a Manager object, then the getSalary method of the Manager class is called instead. The virtual machine knows about the actual type of the object to which e refers, and therefore can invoke the correct method. The fact that an object variable (such as the variable e) can refer to multiple actual types is called polymorphism. Automatically selecting the appropriate method at runtime is called dynamic binding. We discuss both topics in more detail in this chapter. C++ NOTE: In C++, you need to declare a member function as virtual if you want dynamic binding. In Java, dynamic binding is the default behavior; if you do not want a method to be virtual, you tag it as final. (We discuss the final keyword later in this chapter.) Listing 5.1 contains a program that shows how the salary computation differs for Employee (Listing 5.2) and Manager (Listing 5.3) objects. From the Library of Hristo Dimov Hristov 214 Chapter 5 Inheritance Listing 5.1 inheritance/ManagerTest.java 1 package inheritance; 2 3 4 5 6 7 8 9 10 11 12 13 14 /** * This program demonstrates inheritance. * @version 1.21 2004-02-21 * @author Cay Horstmann */ public class ManagerTest { public static void main(String[] args) { // construct a Manager object var boss = new Manager("Carl Cracker", 80000, 1987, 12, 15); boss.setBonus(5000); 15 var staff = new Employee[3]; 16 17 // fill the staff array with Manager and Employee objects 18 19 staff[0] = boss; staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1); staff[2] = new Employee("Tommy Tester", 40000, 1990, 3, 15); 20 21 22 23 // print out information about all Employee objects for (Employee e : staff) System.out.println("name=" + e.getName() + ",salary=" + e.getSalary()); 24 25 26 } 27 28 } Listing 5.2 inheritance/Employee.java 1 package inheritance; 2 3 import java.time.*; 4 5 6 7 8 9 public class Employee { private String name; private double salary; private LocalDate hireDay; 10 11 12 13 14 public Employee(String name, double salary, int year, int month, int day) { this.name = name; this.salary = salary; From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 215 hireDay = LocalDate.of(year, month, day); 15 } 16 17 public String getName() { return name; } 18 19 20 21 22 public double getSalary() { return salary; } 23 24 25 26 27 public LocalDate getHireDay() { return hireDay; } 28 29 30 31 32 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 33 34 35 36 37 38 } Listing 5.3 inheritance/Manager.java 1 package inheritance; 2 3 4 5 public class Manager extends Employee { private double bonus; 6 7 8 9 10 11 12 13 14 15 16 17 18 /** * @param name the employee's name * @param salary the salary * @param year the hire year * @param month the hire month * @param day the hire day */ public Manager(String name, double salary, int year, int month, int day) { super(name, salary, year, month, day); bonus = 0; } 19 (Continues) From the Library of Hristo Dimov Hristov 216 Chapter 5 Inheritance Listing 5.3 (Continued) public double getSalary() { double baseSalary = super.getSalary(); return baseSalary + bonus; } 20 21 22 23 24 25 public void setBonus(double b) { bonus = b; } 26 27 28 29 30 } 5.1.4 Inheritance Hierarchies Inheritance need not stop at deriving one layer of classes. We could have an Executive class that extends Manager, for example. The collection of all classes extending a common superclass is called an inheritance hierarchy, as shown in Figure 5.1. The path from a particular class to its ancestors in the inheritance hierarchy is its inheritance chain. Figure 5.1 Employee inheritance hierarchy From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 217 There is usually more than one chain of descent from a distant ancestor class. You could form subclasses Programmer or Secretary that extend Employee, and they would have nothing to do with the Manager class (or with each other). This process can continue as long as is necessary. C++ NOTE: In C++, a class can have multiple superclasses. Java does not support multiple inheritance. For ways to recover much of the functionality of multiple inheritance, see Section 6.1, “Interfaces,” on p. 296. 5.1.5 Polymorphism A simple rule can help you decide whether or not inheritance is the right design for your data. The “is–a” rule states that every object of the subclass is an object of the superclass. For example, every manager is an employee. Thus, it makes sense for the Manager class to be a subclass of the Employee class. Naturally, the opposite is not true—not every employee is a manager. Another way of formulating the “is–a” rule is the substitution principle. That principle states that you can use a subclass object whenever the program expects a superclass object. For example, you can assign a subclass object to a superclass variable. Employee e; e = new Employee(. . .); // Employee object expected e = new Manager(. . .); // OK, Manager can be used as well In the Java programming language, object variables are polymorphic. A variable of type Employee can refer to an object of type Employee or to an object of any subclass of the Employee class (such as Manager, Executive, Secretary, and so on). We took advantage of this principle in Listing 5.1: Manager boss = new Manager(. . .); Employee[] staff = new Employee[3]; staff[0] = boss; In this case, the variables staff[0] and boss refer to the same object. However, staff[0] is considered to be only an Employee object by the compiler. That means you can call boss.setBonus(5000); // OK but you can’t call staff[0].setBonus(5000); // ERROR From the Library of Hristo Dimov Hristov 218 Chapter 5 Inheritance The declared type of staff[0] is Employee, and the setBonus method is not a method of the Employee class. However, you cannot assign a superclass reference to a subclass variable. For example, it is not legal to make the assignment Manager m = staff[i]; // ERROR The reason is clear: Not all employees are managers. If this assignment were to succeed and m were to refer to an Employee object that is not a manager, then it would later be possible to call m.setBonus(. . .) and a runtime error would occur. CAUTION: In Java, arrays of subclass references can be converted to arrays of superclass references without a cast. For example, consider this array of managers: Manager[] managers = new Manager[10]; It is legal to convert this array to an Employee[] array: Employee[] staff = managers; // OK Sure, why not, you may think. After all, if managers[i] is a Manager, it is also an Employee. But actually, something surprising is going on. Keep in mind that managers and staff are references to the same array. Now consider the statement staff[0] = new Employee("Harry Hacker", . . .); The compiler will cheerfully allow this assignment. But staff[0] and managers[0] are the same reference, so it looks as if we managed to smuggle a mere employee into the management ranks. That would be very bad—calling managers[0].setBonus(1000) would try to access a nonexistent instance field and would corrupt neighboring memory. To make sure no such corruption can occur, all arrays remember the element type with which they were created, and they monitor that only compatible references are stored into them. For example, the array created as new Manager[10] remembers that it is an array of managers. Attempting to store an Employee reference causes an ArrayStoreException. 5.1.6 Understanding Method Calls It is important to understand exactly how a method call is applied to an object. Let’s say we call x.f(args), and the implicit parameter x is declared to be an object of class C. Here is what happens: From the Library of Hristo Dimov Hristov 5.1 1. Classes, Superclasses, and Subclasses 219 The compiler looks at the declared type of the object and the method name. Note that there may be multiple methods, all with the same name, f, but with different parameter types. For example, there may be a method f(int) and a method f(String). The compiler enumerates all methods called f in the class C and all accessible methods called f in the superclasses of C. (Private methods of the superclass are not accessible.) Now the compiler knows all possible candidates for the method to be called. 2. Next, the compiler determines the types of the arguments supplied in the method call. If among all the methods called f there is a unique method whose parameter types are a best match for the supplied arguments, that method is chosen to be called. This process is called overloading resolution. For example, in a call x.f("Hello"), the compiler picks f(String) and not f(int). The situation can get complex because of type conversions (int to double, Manager to Employee, and so on). If the compiler cannot find any method with matching parameter types or if multiple methods all match after applying conversions, the compiler reports an error. Now the compiler knows the name and parameter types of the method that needs to be called. NOTE: Recall that the name and parameter type list for a method is called the method’s signature. For example, f(int) and f(String) are two methods with the same name but different signatures. If you define a method in a subclass that has the same signature as a superclass method, you override the superclass method. The return type is not part of the signature. However, when you override a method, you need to keep the return type compatible. A subclass may change the return type to a subtype of the original type. For example, suppose the Employee class has a method public Employee getBuddy() { . . . } A manager would never want to have a lowly employee as a buddy. To reflect that fact, the Manager subclass can override this method as public Manager getBuddy() { . . . } // OK to change return type We say that the two getBuddy methods have covariant return types. From the Library of Hristo Dimov Hristov 220 Chapter 5 3. 4. Inheritance If the method is private, static, final, or a constructor, then the compiler knows exactly which method to call. (The final modifier is explained in the next section.) This is called static binding. Otherwise, the method to be called depends on the actual type of the implicit parameter, and dynamic binding must be used at runtime. In our example, the compiler would generate an instruction to call f(String) with dynamic binding. When the program runs and uses dynamic binding to call a method, the virtual machine must call the version of the method that is appropriate for the actual type of the object to which x refers. Let’s say the actual type is D, a subclass of C. If the class D defines a method f(String), that method is called. If not, D’s superclass is searched for a method f(String), and so on. It would be time-consuming to carry out this search every time a method is called. Instead, the virtual machine precomputes for each class a method table that lists all method signatures and the actual methods to be called. When a method is actually called, the virtual machine simply makes a table lookup. In our example, the virtual machine consults the method table for the class D and looks up the method to call for f(String). That method may be D.f(String) or X.f(String), where X is some superclass of D. There is one twist to this scenario. If the call is super.f(param), then the compiler consults the method table of the superclass of the implicit parameter. Let’s look at this process in detail in the call e.getSalary() in Listing 5.1. The declared type of e is Employee. The Employee class has a single method, called getSalary, with no method parameters. Therefore, in this case, we don’t worry about overloading resolution. The getSalary method is not private, static, or final, so it is dynamically bound. The virtual machine produces method tables for the Employee and Manager classes. The Employee table shows that all methods are defined in the Employee class itself: Employee: getName() -> Employee.getName() getSalary() -> Employee.getSalary() getHireDay() -> Employee.getHireDay() raiseSalary(double) -> Employee.raiseSalary(double) Actually, that isn’t the whole story—as you will see later in this chapter, the Employee class has a superclass Object from which it inherits a number of methods. We ignore the Object methods for now. The Manager method table is slightly different. Three methods are inherited, one method is redefined, and one method is added. From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 221 Manager: getName() -> Employee.getName() getSalary() -> Manager.getSalary() getHireDay() -> Employee.getHireDay() raiseSalary(double) -> Employee.raiseSalary(double) setBonus(double) -> Manager.setBonus(double) At runtime, the call e.getSalary() is resolved as follows: 1. 2. 3. First, the virtual machine fetches the method table for the actual type of e. That may be the table for Employee, Manager, or another subclass of Employee. Then, the virtual machine looks up the defining class for the getSalary() signature. Now it knows which method to call. Finally, the virtual machine calls the method. Dynamic binding has a very important property: It makes programs extensible without the need for modifying existing code. Suppose a new class Executive is added and there is the possibility that the variable e refers to an object of that class. The code containing the call e.getSalary() need not be recompiled. The Executive.getSalary() method is called automatically if e happens to refer to an object of type Executive. CAUTION: When you override a method, the subclass method must be at least as visible as the superclass method. In particular, if the superclass method is public, the subclass method must also be declared public. It is a common error to accidentally omit the public specifier for the subclass method. The compiler then complains that you try to supply a more restrictive access privilege. 5.1.7 Preventing Inheritance: Final Classes and Methods Occasionally, you want to prevent someone from forming a subclass of one of your classes. Classes that cannot be extended are called final classes, and you use the final modifier in the definition of the class to indicate this. For example, suppose we want to prevent others from subclassing the Executive class. Simply declare the class using the final modifier, as follows: public final class Executive extends Manager { . . . } You can also make a specific method in a class final. If you do this, then no subclass can override that method. (All methods in a final class are automatically final.) For example: From the Library of Hristo Dimov Hristov 222 Chapter 5 Inheritance public class Employee { . . . public final String getName() { return name; } . . . } NOTE: Recall that fields can also be declared as final. A final field cannot be changed after the object has been constructed. However, if a class is declared final, only the methods, not the fields, are automatically final. There is only one good reason to make a method or class final: to make sure its semantics cannot be changed in a subclass. For example, the getTime and setTime methods of the Calendar class are final. This indicates that the designers of the Calendar class have taken over responsibility for the conversion between the Date class and the calendar state. No subclass should be allowed to mess up this arrangement. Similarly, the String class is a final class. That means nobody can define a subclass of String. In other words, if you have a String reference, you know it refers to a String and nothing but a String. Some programmers believe that you should declare all methods as final unless you have a good reason to want polymorphism. In fact, in C++ and C#, methods do not use polymorphism unless you specifically request it. That may be a bit extreme, but we agree that it is a good idea to think carefully about final methods and classes when you design a class hierarchy. In the early days of Java, some programmers used the final keyword hoping to avoid the overhead of dynamic binding. If a method is not overridden, and it is short, then a compiler can optimize the method call away—a process called inlining. For example, inlining the call e.getName() replaces it with the field access e.name. This is a worthwhile improvement—CPUs hate branching because it interferes with their strategy of prefetching instructions while processing the current one. However, if getName can be overridden in another class, then the compiler cannot inline it because it has no way of knowing what the overriding code may do. Fortunately, the just-in-time compiler in the virtual machine can do a better job than a traditional compiler. It knows exactly which classes extend a given class, and it can check whether any class actually overrides a given method. If a method is short, frequently called, and not actually overridden, the justin-time compiler can inline it. What happens if the virtual machine loads From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 223 another subclass that overrides an inlined method? Then the optimizer must undo the inlining. That takes time, but it happens rarely. 5.1.8 Casting Recall from Chapter 3 that the process of forcing a conversion from one type to another is called casting. The Java programming language has a special notation for casts. For example, double x = 3.405; int nx = (int) x; converts the value of the expression x into an integer, discarding the fractional part. Just as you occasionally need to convert a floating-point number to an integer, you may need to convert an object reference from one class to another. To actually make a cast of an object reference, use a syntax similar to what you use for casting numeric expressions. Surround the target class name with parentheses and place it before the object reference you want to cast. For example: Manager boss = (Manager) staff[0]; There is only one reason why you would want to make a cast—to use an object in its full capacity after its actual type has been temporarily forgotten. For example, in the ManagerTest class, the staff array had to be an array of Employee objects because some of its elements were regular employees. We would need to cast the managerial elements of the array back to Manager to access any of its new variables. (Note that in the sample code for the first section, we made a special effort to avoid the cast. We initialized the boss variable with a Manager object before storing it in the array. We needed the correct type to set the bonus of the manager.) As you know, in Java every variable has a type. The type describes the kind of object the variable refers to and what it can do. For example, staff[i] refers to an Employee object (so it can also refer to a Manager object). The compiler checks that you do not promise too much when you store a value in a variable. If you assign a subclass reference to a superclass variable, you are promising less, and the compiler will simply let you do it. If you assign a superclass reference to a subclass variable, you are promising more. Then you must use a cast so that your promise can be checked at runtime. What happens if you try to cast down an inheritance chain and are “lying” about what an object contains? Manager boss = (Manager) staff[1]; // ERROR From the Library of Hristo Dimov Hristov 224 Chapter 5 Inheritance When the program runs, the Java runtime system notices the broken promise and generates a ClassCastException. If you do not catch the exception, your program terminates. Thus, it is good programming practice to find out whether a cast will succeed before attempting it. Simply use the instanceof operator. For example: if (staff[1] instanceof Manager) { boss = (Manager) staff[1]; . . . } Finally, the compiler will not let you make a cast if there is no chance for the cast to succeed. For example, the cast String c = (String) staff[1]; is a compile-time error because String is not a subclass of Employee. To sum up: • You can cast only within an inheritance hierarchy. • Use instanceof to check before casting from a superclass to a subclass. NOTE: The test x instanceof C does not generate an exception if x is null. It simply returns false. That makes sense: null refers to no object, so it certainly doesn’t refer to an object of type C. Actually, converting the type of an object by a cast is not usually a good idea. In our example, you do not need to cast an Employee object to a Manager object for most purposes. The getSalary method will work correctly on both objects of both classes. The dynamic binding that makes polymorphism work locates the correct method automatically. The only reason to make the cast is to use a method that is unique to managers, such as setBonus. If for some reason you find yourself wanting to call setBonus on Employee objects, ask yourself whether this is an indication of a design flaw in the superclass. It may make sense to redesign the superclass and add a setBonus method. Remember, it takes only one uncaught ClassCastException to terminate your program. In general, it is best to minimize the use of casts and the instanceof operator. From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 225 C++ NOTE: Java uses the cast syntax from the “bad old days” of C, but it works like the safe dynamic_cast operation of C++. For example, Manager boss = (Manager) staff[1]; // Java is the same as Manager* boss = dynamic_cast<Manager*>(staff[1]); // C++ with one important difference. If the cast fails, it does not yield a null object but throws an exception. In this sense, it is like a C++ cast of references. This is a pain in the neck. In C++, you can take care of the type test and type conversion in one operation. Manager* boss = dynamic_cast<Manager*>(staff[1]); // C++ if (boss != NULL) . . . In Java, you need to use a combination of the instanceof operator and a cast. if (staff[1] instanceof Manager) { Manager boss = (Manager) staff[1]; . . . } 5.1.9 Abstract Classes As you move up the inheritance hierarchy, classes become more general and probably more abstract. At some point, the ancestor class becomes so general that you think of it more as a basis for other classes than as a class with specific instances you want to use. Consider, for example, an extension of our Employee class hierarchy. An employee is a person, and so is a student. Let us extend our class hierarchy to include classes Person and Student. Figure 5.2 shows the inheritance relationships between these classes. Why bother with so high a level of abstraction? There are some attributes that make sense for every person, such as name. Both students and employees have names, and introducing a common superclass lets us factor out the getName method to a higher level in the inheritance hierarchy. Now let’s add another method, getDescription, whose purpose is to return a brief description of the person, such as an employee with a salary of $50,000.00 a student majoring in computer science It is easy to implement this method for the Employee and Student classes. But what information can you provide in the Person class? The Person class knows nothing about the person except the name. Of course, you could implement From the Library of Hristo Dimov Hristov 226 Chapter 5 Inheritance Figure 5.2 Inheritance diagram for Person and its subclasses Person.getDescription() to return an empty string. But there is a better way. If you use the abstract keyword, you do not need to implement the method at all. public abstract String getDescription(); // no implementation required For added clarity, a class with one or more abstract methods must itself be declared abstract. public abstract class Person { . . . public abstract String getDescription(); } In addition to abstract methods, abstract classes can have fields and concrete methods. For example, the Person class stores the name of the person and has a concrete method that returns it. public abstract class Person { private String name; public Person(String name) { this.name = name; } public abstract String getDescription(); public String getName() { From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 227 return name; } } TIP: Some programmers don’t realize that abstract classes can have concrete methods. You should always move common fields and methods (whether abstract or not) to the superclass (whether abstract or not). Abstract methods act as placeholders for methods that are implemented in the subclasses. When you extend an abstract class, you have two choices. You can leave some or all of the abstract methods undefined; then you must tag the subclass as abstract as well. Or you can define all methods, and the subclass is no longer abstract. For example, we will define a Student class that extends the abstract Person class and implements the getDescription method. None of the methods of the Student class are abstract, so it does not need to be declared as an abstract class. A class can even be declared as abstract though it has no abstract methods. Abstract classes cannot be instantiated. That is, if a class is declared as abstract, no objects of that class can be created. For example, the expression new Person("Vince Vu") is an error. However, you can create objects of concrete subclasses. Note that you can still create object variables of an abstract class, but such a variable must refer to an object of a nonabstract subclass. For example: Person p = new Student("Vince Vu", "Economics"); Here p is a variable of the abstract type Person that refers to an instance of the nonabstract subclass Student. C++ NOTE: In C++, an abstract method is called a pure virtual function and is tagged with a trailing = 0, such as in class Person // C++ { public: virtual string getDescription() = 0; . . . }; A C++ class is abstract if it has at least one pure virtual function. In C++, there is no special keyword to denote abstract classes. From the Library of Hristo Dimov Hristov 228 Chapter 5 Inheritance Let us define a concrete subclass Student that extends the abstract class Person: public class Student extends Person { private String major; public Student(String name, String major) { super(name); this.major = major; } public String getDescription() { return "a student majoring in " + major; } } The Student class defines the getDescription method. Therefore, all methods in the Student class are concrete, and the class is no longer an abstract class. The program shown in Listing 5.4 defines the abstract superclass Person (Listing 5.5) and two concrete subclasses, Employee (Listing 5.6) and Student (Listing 5.7). We fill an array of Person references with employee and student objects: var people = new Person[2]; people[0] = new Employee(. . .); people[1] = new Student(. . .); We then print the names and descriptions of these objects: for (Person p : people) System.out.println(p.getName() + ", " + p.getDescription()); Some people are baffled by the call p.getDescription() Isn’t this a call to an undefined method? Keep in mind that the variable p never refers to a Person object because it is impossible to construct an object of the abstract Person class. The variable p always refers to an object of a concrete subclass such as Employee or Student. For these objects, the getDescription method is defined. Could you have omitted the abstract method altogether from the Person superclass, simply defining the getDescription methods in the Employee and Student subclasses? If you did that, you wouldn’t have been able to invoke the getDescription method on the variable p. The compiler ensures that you invoke only methods that are declared in the class. From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 229 Abstract methods are an important concept in the Java programming language. You will encounter them most commonly inside interfaces. For more information about interfaces, turn to Chapter 6. Listing 5.4 abstractClasses/PersonTest.java 1 package abstractClasses; 2 3 4 5 6 7 8 9 10 11 12 /** * This program demonstrates abstract classes. * @version 1.01 2004-02-21 * @author Cay Horstmann */ public class PersonTest { public static void main(String[] args) { var people = new Person[2]; 13 // fill the people array with Student and Employee objects people[0] = new Employee("Harry Hacker", 50000, 1989, 10, 1); people[1] = new Student("Maria Morris", "computer science"); 14 15 16 17 // print out names and descriptions of all Person objects for (Person p : people) System.out.println(p.getName() + ", " + p.getDescription()); 18 19 20 } 21 22 } Listing 5.5 abstractClasses/Person.java 1 package abstractClasses; 2 3 4 5 6 public abstract class Person { public abstract String getDescription(); private String name; 7 public Person(String name) { this.name = name; } 8 9 10 11 12 public String getName() { return name; } 13 14 15 16 17 } From the Library of Hristo Dimov Hristov 230 Chapter 5 Inheritance Listing 5.6 abstractClasses/Employee.java 1 package abstractClasses; 2 3 import java.time.*; 4 5 6 7 8 public class Employee extends Person { private double salary; private LocalDate hireDay; 9 public Employee(String name, double salary, int year, int month, int day) { super(name); this.salary = salary; hireDay = LocalDate.of(year, month, day); } 10 11 12 13 14 15 16 public double getSalary() { return salary; } 17 18 19 20 21 public LocalDate getHireDay() { return hireDay; } 22 23 24 25 26 public String getDescription() { return String.format("an employee with a salary of $%.2f", salary); } 27 28 29 30 31 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 32 33 34 35 36 37 } Listing 5.7 abstractClasses/Student.java 1 package abstractClasses; 2 3 4 5 public class Student extends Person { private String major; 6 From the Library of Hristo Dimov Hristov 5.1 Classes, Superclasses, and Subclasses 231 /** * @param name the student's name * @param major the student's major */ public Student(String name, String major) { // pass name to superclass constructor super(name); this.major = major; } 7 8 9 10 11 12 13 14 15 16 17 public String getDescription() { return "a student majoring in " + major; } 18 19 20 21 22 } 5.1.10 Protected Access As you know, fields in a class are best tagged as private, and methods are usually tagged as public. Any features declared private won’t be accessible in other classes. As we said at the beginning of this chapter, this is also true for subclasses: A subclass cannot access the private fields of its superclass. There are times, however, when you want to restrict a method to subclasses only or, less commonly, to allow subclass methods to access a superclass field. In that case, you declare a class feature as protected. For example, if the superclass Employee declares the hireDay field as protected instead of private, then the Manager methods can access it directly. In Java, a protected field is accessible by any class in the same package. Now consider an Administrator subclass in a different package. The methods of the Administrator class can peek inside the hireDay field of Administrator objects only, not of other Employee objects. This restriction is made so that you can’t abuse the protected mechanism by forming subclasses just to gain access to the protected fields. In practice, use protected fields with caution. Suppose your class is used by other programmers and you designed it with protected fields. Unknown to you, other programmers may inherit classes from your class and start accessing your protected fields. In this case, you can no longer change the implementation of your class without upsetting those programmers. That is against the spirit of OOP, which encourages data encapsulation. Protected methods make more sense. A class may declare a method as protected if it is tricky to use. This indicates that the subclasses (which, presumably, From the Library of Hristo Dimov Hristov 232 Chapter 5 Inheritance know their ancestor well) can be trusted to use the method correctly, but other classes cannot. A good example of this kind of method is the clone method of the Object class—see Chapter 6 for more details. C++ NOTE: As already mentioned, protected features in Java are accessible to all subclasses as well as to all other classes in the same package. This is slightly different from the C++ meaning of protected, and it makes the notion of protected in Java even less safe than in C++. Here is a summary of the four access control modifiers in Java: 1. Accessible in the class only (private). 2. 3. 4. Accessible by the world (public). Accessible in the package and all subclasses (protected). Accessible in the package—the (unfortunate) default. No modifiers are needed. 5.2 Object: The Cosmic Superclass The Object class is the ultimate ancestor—every class in Java extends Object. However, you never have to write public class Employee extends Object The ultimate superclass Object is taken for granted if no superclass is explicitly mentioned. Since every class in Java extends Object, it is important to be familiar with the services provided by the Object class. We go over the basic ones in this chapter; consult the later chapters or view the online documentation for what is not covered here. (Several methods of Object come up only when dealing with concurrency—see Chapter 12.) 5.2.1 Variables of Type Object You can use a variable of type Object to refer to objects of any type: Object obj = new Employee("Harry Hacker", 35000); Of course, a variable of type Object is only useful as a generic holder for arbitrary values. To do anything specific with the value, you need to have some knowledge about the original type and apply a cast: From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 233 Employee e = (Employee) obj; In Java, only the values of primitive types (numbers, characters, and boolean values) are not objects. All array types, no matter whether they are arrays of objects or arrays of primitive types, are class types that extend the Object class. Employee[] staff = new Employee[10]; obj = staff; // OK obj = new int[10]; // OK C++ NOTE: In C++, there is no cosmic root class. However, every pointer can be converted to a void* pointer. 5.2.2 The equals Method The equals method in the Object class tests whether one object is considered equal to another. The equals method, as implemented in the Object class, determines whether two object references are identical. This is a pretty reasonable default—if two objects are identical, they should certainly be equal. For quite a few classes, nothing else is required. For example, it makes little sense to compare two PrintStream objects for equality. However, you will often want to implement state-based equality testing, in which two objects are considered equal when they have the same state. For example, let us consider two employees equal if they have the same name, salary, and hire date. (In an actual employee database, it would be more sensible to compare IDs instead. We use this example to demonstrate the mechanics of implementing the equals method.) public class Employee { . . . public boolean equals(Object otherObject) { // a quick test to see if the objects are identical if (this == otherObject) return true; // must return false if the explicit parameter is null if (otherObject == null) return false; // if the classes don't match, they can't be equal if (getClass() != otherObject.getClass()) return false; // now we know otherObject is a non-null Employee From the Library of Hristo Dimov Hristov 234 Chapter 5 Inheritance Employee other = (Employee) otherObject; // test whether the fields have identical values return name.equals(other.name) && salary == other.salary && hireDay.equals(other.hireDay); } } The getClass method returns the class of an object—we discuss this method in detail later in this chapter. In our test, two objects can only be equal when they belong to the same class. TIP: To guard against the possibility that name or hireDay are null, use the Objects.equals method. The call Objects.equals(a, b) returns true if both arguments are null, false if only one is null, and calls a.equals(b) otherwise. With that method, the last statement of the Employee.equals method becomes return Objects.equals(name, other.name) && salary == other.salary && Objects.equals(hireDay, other.hireDay); When you define the equals method for a subclass, first call equals on the superclass. If that test doesn’t pass, then the objects can’t be equal. If the superclass fields are equal, you are ready to compare the instance fields of the subclass. public class Manager extends Employee { . . . public boolean equals(Object otherObject) { if (!super.equals(otherObject)) return false; // super.equals checked that this and otherObject belong to the same class Manager other = (Manager) otherObject; return bonus == other.bonus; } } 5.2.3 Equality Testing and Inheritance How should the equals method behave if the implicit and explicit parameters don’t belong to the same class? This has been an area of some controversy. In the preceding example, the equals method returns false if the classes don’t match exactly. But many programmers use an instanceof test instead: if (!(otherObject instanceof Employee)) return false; From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 235 This leaves open the possibility that otherObject can belong to a subclass. However, this approach can get you into trouble. Here is why. The Java Language Specification requires that the equals method has the following properties: 1. 2. 3. 4. 5. It is reflexive: For any non-null reference x, x.equals(x) should return true. It is symmetric: For any references x and y, x.equals(y) should return true if and only if y.equals(x) returns true. It is transitive: For any references x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. It is consistent: If the objects to which x and y refer haven’t changed, then repeated calls to x.equals(y) return the same value. For any non-null reference x, x.equals(null) should return false. These rules are certainly reasonable. You wouldn’t want a library implementor to ponder whether to call x.equals(y) or y.equals(x) when locating an element in a data structure. However, the symmetry rule has subtle consequences when the parameters belong to different classes. Consider a call e.equals(m) where e is an Employee object and m is a Manager object, both of which happen to have the same name, salary, and hire date. If Employee.equals uses an instanceof test, the call returns true. But that means that the reverse call m.equals(e) also needs to return true—the symmetry rule does not allow it to return false or to throw an exception. That leaves the Manager class in a bind. Its equals method must be willing to compare itself to any Employee, without taking manager-specific information into account! All of a sudden, the instanceof test looks less attractive. Some authors have gone on record that the getClass test is wrong because it violates the substitution principle. A commonly cited example is the equals method in the AbstractSet class that tests whether two sets have the same elements. The AbstractSet class has two concrete subclasses, TreeSet and HashSet, that use different algorithms for locating set elements. You really want to be able to compare any two sets, no matter how they are implemented. However, the set example is rather specialized. It would make sense to declare AbstractSet.equals as final, because nobody should redefine the semantics of set From the Library of Hristo Dimov Hristov 236 Chapter 5 Inheritance equality. (The method is not actually final. This allows a subclass to implement a more efficient algorithm for the equality test.) The way we see it, there are two distinct scenarios: • If subclasses can have their own notion of equality, then the symmetry requirement forces you to use the getClass test. • If the notion of equality is fixed in the superclass, then you can use the instanceof test and allow objects of different subclasses to be equal to one another. In the example with employees and managers, we consider two objects to be equal when they have matching fields. If we have two Manager objects with the same name, salary, and hire date, but with different bonuses, we want them to be different. Therefore, we use the getClass test. But suppose we used an employee ID for equality testing. This notion of equality makes sense for all subclasses. Then we could use the instanceof test, and we should have declared Employee.equals as final. NOTE: The standard Java library contains over 150 implementations of equals methods, with a mishmash of using instanceof, calling getClass, catching a ClassCastException, or doing nothing at all. Check out the API documentation of the java.sql.Timestamp class, where the implementors note with some embarrassment that they have painted themselves in a corner. The Timestamp class inherits from java.util.Date, whose equals method uses an instanceof test, and it is impossible to override equals to be both symmetric and accurate. Here is a recipe for writing the perfect equals method: 1. 2. Name the explicit parameter otherObject—later, you will need to cast it to another variable that you should call other. Test whether this happens to be identical to otherObject: if (this == otherObject) return true; This statement is just an optimization. In practice, this is a common case. It is much cheaper to check for identity than to compare the fields. 3. Test whether otherObject is null and return false if it is. This test is required. if (otherObject == null) return false; 4. Compare the classes of this and otherObject. If the semantics of equals can change in subclasses, use the getClass test: if (getClass() != otherObject.getClass()) return false; From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 237 If the same semantics holds for all subclasses, you can use an instanceof test: if (!(otherObject instanceof ClassName)) return false; 5. Cast otherObject to a variable of your class type: ClassName other = (ClassName) otherObject 6. Now compare the fields, as required by your notion of equality. Use == for primitive type fields, Objects.equals for object fields. Return true if all fields match, false otherwise. return field1 == other.field1 && Objects.equals(field2, other.field2) && . . .; If you redefine equals in a subclass, include a call to super.equals(other). TIP: If you have fields of array type, you can use the static Arrays.equals method to check that the corresponding array elements are equal. CAUTION: Here is a common mistake when implementing the equals method. Can you spot the problem? public class Employee { public boolean equals(Employee other) { return other != null && getClass() == other.getClass() && Objects.equals(name, other.name) && salary == other.salary && Objects.equals(hireDay, other.hireDay); } . . . } This method declares the explicit parameter type as Employee. As a result, it does not override the equals method of the Object class but defines a completely unrelated method. You can protect yourself against this type of error by tagging methods that are intended to override superclass methods with @Override: @Override public boolean equals(Object other) From the Library of Hristo Dimov Hristov 238 Chapter 5 Inheritance If you made a mistake and are defining a new method, the compiler reports an error. For example, suppose you add the following declaration to the Employee class: @Override public boolean equals(Employee other) An error is reported because this method doesn’t override any method from the Object superclass. java.util.Arrays 1.2 • static boolean equals(xxx[] a, xxx[] b) 5 returns true if the arrays have equal lengths and equal elements in corresponding positions. The component type xxx of the array can be Object, int, long, short, char, byte, boolean, float, or double. java.util.Objects 7 • static boolean equals(Object a, Object b) returns true if a and b are both null, false if exactly one of them is null, and a.equals(b) otherwise. 5.2.4 The hashCode Method A hash code is an integer that is derived from an object. Hash codes should be scrambled—if x and y are two distinct objects, there should be a high probability that x.hashCode() and y.hashCode() are different. Table 5.1 lists a few examples of hash codes that result from the hashCode method of the String class. Table 5.1 Hash Codes Resulting from the hashCode Method String Hash Code Hello 69609650 Harry 69496448 Hacker -2141031506 The String class uses the following algorithm to compute the hash code: int hash = 0; for (int i = 0; i < length(); i++) hash = 31 * hash + charAt(i); From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 239 The hashCode method is defined in the Object class. Therefore, every object has a default hash code. That hash code is derived from the object’s memory address. Consider this example: var s = "Ok"; var sb = new StringBuilder(s); System.out.println(s.hashCode() + " " + sb.hashCode()); var t = new String("Ok"); var tb = new StringBuilder(t); System.out.println(t.hashCode() + " " + tb.hashCode()); Table 5.2 shows the result. Table 5.2 Hash Codes of Strings and String Builders Object Hash Code Object Hash Code s 2556 t 2556 sb 20526976 tb 20527144 Note that the strings s and t have the same hash code because, for strings, the hash codes are derived from their contents. The string builders sb and tb have different hash codes because no hashCode method has been defined for the StringBuilder class and the default hashCode method in the Object class derives the hash code from the object’s memory address. If you redefine the equals method, you will also need to redefine the hashCode method for objects that users might insert into a hash table. (We discuss hash tables in Chapter 9.) The hashCode method should return an integer (which can be negative). Just combine the hash codes of the instance fields so that the hash codes for different objects are likely to be widely scattered. For example, here is a hashCode method for the Employee class: public class Employee { public int hashCode() { return 7 * name.hashCode() + 11 * new Double(salary).hashCode() + 13 * hireDay.hashCode(); } . . . } From the Library of Hristo Dimov Hristov 240 Chapter 5 Inheritance However, you can do better. First, use the null-safe method Objects.hashCode. It returns 0 if its argument is null and the result of calling hashCode on the argument otherwise. Also, use the static Double.hashCode method to avoid creating a Double object: public int hashCode() { return 7 * Objects.hashCode(name) + 11 * Double.hashCode(salary) + 13 * Objects.hashCode(hireDay); } Even better, when you need to combine multiple hash values, call Objects.hash with all of them. It will call Objects.hashCode for each argument and combine the values. Then the Employee.hashCode method is simply public int hashCode() { return Objects.hash(name, salary, hireDay); } Your definitions of equals and hashCode must be compatible: If x.equals(y) is true, then x.hashCode() must return the same value as y.hashCode(). For example, if you define Employee.equals to compare employee IDs, then the hashCode method needs to hash the IDs, not employee names or memory addresses. TIP: If you have fields of an array type, you can use the static Arrays.hashCode method to compute a hash code composed of the hash codes of the array elements. java.lang.Object 1.0 • int hashCode() returns a hash code for this object. A hash code can be any integer, positive or negative. Equal objects need to return identical hash codes. java.util.Objects 7 • static int hash(Object... objects) returns a hash code that is combined from the hash codes of all supplied objects. • static int hashCode(Object a) returns 0 if a is null or a.hashCode() otherwise. From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 241 java.lang.(Integer|Long|Short|Byte|Double|Float|Character|Boolean) 1.0 • static int hashCode(xxx value) 8 returns the hash code of the given value. Here xxx is the primitive type corresponding to the given wrapper type. java.util.Arrays 1.2 • static int hashCode(xxx[] a) 5 computes the hash code of the array a. The component type xxx of the array can be Object, int, long, short, char, byte, boolean, float, or double. 5.2.5 The toString Method Another important method in Object is the toString method that returns a string representing the value of this object. Here is a typical example. The toString method of the Point class returns a string like this: java.awt.Point[x=10,y=20] Most (but not all) toString methods follow this format: the name of the class, then the field values enclosed in square brackets. Here is an implementation of the toString method for the Employee class: public String toString() { return "Employee[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]"; } Actually, you can do a little better. Instead of hardwiring the class name into the toString method, call getClass().getName() to obtain a string with the class name. public String toString() { return getClass().getName() + "[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]"; } Such toString method will also work for subclasses. From the Library of Hristo Dimov Hristov 242 Chapter 5 Inheritance Of course, the subclass programmer should define its own toString method and add the subclass fields. If the superclass uses getClass().getName(), then the subclass can simply call super.toString(). For example, here is a toString method for the Manager class: public class Manager extends Employee { . . . public String toString() { return super.toString() + "[bonus=" + bonus + "]"; } } Now a Manager object is printed as Manager[name=. . .,salary=. . .,hireDay=. . .][bonus=. . .] The toString method is ubiquitous for an important reason: Whenever an object is concatenated with a string by the “+” operator, the Java compiler automatically invokes the toString method to obtain a string representation of the object. For example: var p = new Point(10, 20); String message = "The current position is " + p; // automatically invokes p.toString() TIP: Instead of writing x.toString(), you can write "" + x. This statement concatenates the empty string with the string representation of x that is exactly x.toString(). Unlike toString, this statement even works if x is of primitive type. If x is any object and you call System.out.println(x); then the println method simply calls x.toString() and prints the resulting string. The Object class defines the toString method to print the class name and the hash code of the object. For example, the call System.out.println(System.out) From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 243 produces an output that looks like this: java.io.PrintStream@2f6684 The reason is that the implementor of the PrintStream class didn’t bother to override the toString method. CAUTION: Annoyingly, arrays inherit the toString method from Object, with the added twist that the array type is printed in an archaic format. For example, int[] luckyNumbers = { 2, 3, 5, 7, 11, 13 }; String s = "" + luckyNumbers; yields the string "[I@1a46e30". (The prefix [I denotes an array of integers.) The remedy is to call the static Arrays.toString method instead. The code String s = Arrays.toString(luckyNumbers); yields the string "[2, 3, 5, 7, 11, 13]". To correctly print multidimensional arrays (that is, arrays of arrays), use Arrays.deepToString. The toString method is a great tool for logging. Many classes in the standard class library define the toString method so that you can get useful information about the state of an object. This is particularly useful in logging messages like this: System.out.println("Current position = " + position); As we explain in Chapter 7, an even better solution is to use an object of the Logger class and call Logger.global.info("Current position = " + position); TIP: We strongly recommend that you add a toString method to each class that you write. You, as well as other programmers who use your classes, will be grateful for the logging support. The program in Listing 5.8 tests the equals, hashCode, and toString methods for the classes Employee (Listing 5.9) and Manager (Listing 5.10). From the Library of Hristo Dimov Hristov 244 Chapter 5 Inheritance Listing 5.8 equals/EqualsTest.java 1 package equals; 2 3 4 5 6 7 8 9 10 11 12 13 14 15 /** * This program demonstrates the equals method. * @version 1.12 2012-01-26 * @author Cay Horstmann */ public class EqualsTest { public static void main(String[] args) { var alice1 = new Employee("Alice Adams", 75000, 1987, 12, 15); var alice2 = alice1; var alice3 = new Employee("Alice Adams", 75000, 1987, 12, 15); var bob = new Employee("Bob Brandson", 50000, 1989, 10, 1); 16 System.out.println("alice1 == alice2: " + (alice1 == alice2)); 17 18 System.out.println("alice1 == alice3: " + (alice1 == alice3)); 19 20 System.out.println("alice1.equals(alice3): " + alice1.equals(alice3)); 21 22 System.out.println("alice1.equals(bob): " + alice1.equals(bob)); 23 24 System.out.println("bob.toString(): " + bob); 25 26 var carl = new Manager("Carl Cracker", 80000, 1987, 12, 15); var boss = new Manager("Carl Cracker", 80000, 1987, 12, 15); boss.setBonus(5000); System.out.println("boss.toString(): " + boss); System.out.println("carl.equals(boss): " + carl.equals(boss)); System.out.println("alice1.hashCode(): " + alice1.hashCode()); System.out.println("alice3.hashCode(): " + alice3.hashCode()); System.out.println("bob.hashCode(): " + bob.hashCode()); System.out.println("carl.hashCode(): " + carl.hashCode()); 27 28 29 30 31 32 33 34 35 } 36 37 } Listing 5.9 equals/Employee.java 1 package equals; 2 3 4 import java.time.*; import java.util.Objects; 5 From the Library of Hristo Dimov Hristov 5.2 6 7 8 9 10 Object: The Cosmic Superclass 245 public class Employee { private String name; private double salary; private LocalDate hireDay; 11 12 13 14 15 16 17 public Employee(String name, double salary, int year, int month, int day) { this.name = name; this.salary = salary; hireDay = LocalDate.of(year, month, day); } 18 19 20 21 22 public String getName() { return name; } 23 24 25 26 27 public double getSalary() { return salary; } 28 29 30 31 32 public LocalDate getHireDay() { return hireDay; } 33 34 35 36 37 38 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 39 40 41 42 43 public boolean equals(Object otherObject) { // a quick test to see if the objects are identical if (this == otherObject) return true; 44 45 46 // must return false if the explicit parameter is null if (otherObject == null) return false; 47 48 49 // if the classes don't match, they can't be equal if (getClass() != otherObject.getClass()) return false; 50 51 52 // now we know otherObject is a non-null Employee var other = (Employee) otherObject; 53 (Continues) From the Library of Hristo Dimov Hristov 246 Chapter 5 Inheritance Listing 5.9 (Continued) // test whether the fields have identical values return Objects.equals(name, other.name) && salary == other.salary && Objects.equals(hireDay, other.hireDay); 54 55 56 } 57 58 public int hashCode() { return Objects.hash(name, salary, hireDay); } 59 60 61 62 63 public String toString() { return getClass().getName() + "[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]"; } 64 65 66 67 68 69 } Listing 5.10 equals/Manager.java 1 package equals; 2 3 4 5 public class Manager extends Employee { private double bonus; 6 7 8 9 10 11 public Manager(String name, double salary, int year, int month, int day) { super(name, salary, year, month, day); bonus = 0; } 12 13 14 15 16 17 public double getSalary() { double baseSalary = super.getSalary(); return baseSalary + bonus; } 18 19 20 21 22 public void setBonus(double bonus) { this.bonus = bonus; } 23 From the Library of Hristo Dimov Hristov 5.2 Object: The Cosmic Superclass 247 public boolean equals(Object otherObject) { if (!super.equals(otherObject)) return false; var other = (Manager) otherObject; // super.equals checked that this and other belong to the same class return bonus == other.bonus; } 24 25 26 27 28 29 30 31 public int hashCode() { return java.util.Objects.hash(super.hashCode(), bonus); } 32 33 34 35 36 public String toString() { return super.toString() + "[bonus=" + bonus + "]"; } 37 38 39 40 41 } java.lang.Object 1.0 • Class getClass() returns a class object that contains information about the object. As you will see later in this chapter, Java has a runtime representation for classes that is encapsulated in the Class class. • boolean equals(Object otherObject) compares two objects for equality; returns true if the objects point to the same area of memory, and false otherwise. You should override this method in your own classes. • String toString() returns a string that represents the value of this object. You should override this method in your own classes. java.lang.Class 1.0 • String getName() returns the name of this class. • Class getSuperclass() returns the superclass of this class as a Class object. From the Library of Hristo Dimov Hristov 248 Chapter 5 Inheritance 5.3 Generic Array Lists In some programming languages—in particular, in C and C++—you have to fix the sizes of all arrays at compile time. Programmers hate this because it forces them into uncomfortable tradeoffs. How many employees will be in a department? Surely no more than 100. What if there is a humongous department with 150 employees? Do we want to waste 90 entries for every department with just 10 employees? In Java, the situation is somewhat better. You can set the size of an array at runtime. int actualSize = . . .; var staff = new Employee[actualSize]; Of course, this code does not completely solve the problem of dynamically modifying arrays at runtime. Once you set the array size, you cannot change it easily. Instead, in Java you can deal with this common situation by using another Java class, called ArrayList. The ArrayList class is similar to an array, but it automatically adjusts its capacity as you add and remove elements, without any additional code. is a generic class with a type parameter. To specify the type of the element objects that the array list holds, you append a class name enclosed in angle brackets, such as ArrayList<Employee>. You will see in Chapter 8 how to define your own generic class, but you don’t need to know any of those technicalities to use the ArrayList type. ArrayList The following sections show you how to work with array lists. 5.3.1 Declaring Array Lists Here is how to declare and construct an array list that holds Employee objects: ArrayList<Employee> staff = new ArrayList<Employee>(); As of Java 10, it is a good idea to use the var keyword to avoid duplicating the class name: var staff = new ArrayList<Employee>(); It you don’t use the var keyword, you can omit the type parameter on the right-hand side: ArrayList<Employee> staff = new ArrayList<>(); This is called the “diamond” syntax because the empty brackets <> resemble a diamond. Use the diamond syntax together with the new operator. The From the Library of Hristo Dimov Hristov 5.3 Generic Array Lists 249 compiler checks what happens to the new value. If it is assigned to a variable, passed into a method, or returned from a method, then the compiler checks the generic type of the variable, parameter, or method. It then places that type into the <>. In our example, the new ArrayList<>() is assigned to a variable of type ArrayList<Employee>. Therefore, the generic type is Employee. CAUTION: If you declare an ArrayList with var, do not use the diamond syntax. The declaration var elements = new ArrayList<>(); yields an ArrayList<Object>. NOTE: Before Java 5, there were no generic classes. Instead, there was a single ArrayList class, a one-size-fits-all collection holding elements of type Object. You can still use ArrayList without a <. . .> suffix. It is considered a “raw” type, with the type parameter erased. NOTE: In even older versions of Java, programmers used the Vector class for dynamic arrays. However, the ArrayList class is more efficient, and there is no longer any good reason to use the Vector class. Use the add method to add new elements to an array list. For example, here is how you populate an array list with Employee objects: staff.add(new Employee("Harry Hacker", . . .)); staff.add(new Employee("Tony Tester", . . .)); The array list manages an internal array of object references. Eventually, that array will run out of space. This is where array lists work their magic: If you call add and the internal array is full, the array list automatically creates a bigger array and copies all the objects from the smaller to the bigger array. If you already know, or have a good guess, how many elements you want to store, call the ensureCapacity method before filling the array list: staff.ensureCapacity(100); That call allocates an internal array of 100 objects. Then, the first 100 calls to add will not involve any costly reallocation. You can also pass an initial capacity to the ArrayList constructor: ArrayList<Employee> staff = new ArrayList<>(100); From the Library of Hristo Dimov Hristov 250 Chapter 5 Inheritance CAUTION: Allocating an array list as new ArrayList<>(100) // capacity is 100 is not the same as allocating a new array as new Employee[100] // size is 100 There is an important distinction between the capacity of an array list and the size of an array. If you allocate an array with 100 entries, then the array has 100 slots, ready for use. An array list with a capacity of 100 elements has the potential of holding 100 elements (and, in fact, more than 100, at the cost of additional reallocations)—but at the beginning, even after its initial construction, an array list holds no elements at all. The size method returns the actual number of elements in the array list. For example, staff.size() returns the current number of elements in the staff array list. This is the equivalent of a.length for an array a. Once you are reasonably sure that the array list is at its permanent size, you can call the trimToSize method. This method adjusts the size of the memory block to use exactly as much storage space as is required to hold the current number of elements. The garbage collector will reclaim any excess memory. Once you trim the size of an array list, adding new elements will move the block again, which takes time. You should only use trimToSize when you are sure you won’t add any more elements to the array list. C++ NOTE: The ArrayList class is similar to the C++ vector template. Both ArrayList and vector are generic types. But the C++ vector template overloads the [] operator for convenient element access. Java does not have operator overloading, so it must use explicit method calls instead. Moreover, C++ vectors are copied by value. If a and b are two vectors, then the assignment a = b makes a into a new vector with the same length as b, and all elements are copied from b to a. The same assignment in Java makes both a and b refer to the same array list. From the Library of Hristo Dimov Hristov 5.3 Generic Array Lists 251 java.util.ArrayList<E> 1.2 • ArrayList<E>() constructs an empty array list. • ArrayList<E>(int initialCapacity) constructs an empty array list with the specified capacity. • boolean add(E obj) appends obj at the end of the array list. Always returns true. • int size() returns the number of elements currently stored in the array list. (Of course, this is never larger than the array list’s capacity.) • void ensureCapacity(int capacity) ensures that the array list has the capacity to store the given number of elements without reallocating its internal storage array. • void trimToSize() reduces the storage capacity of the array list to its current size. 5.3.2 Accessing Array List Elements Unfortunately, nothing comes for free. The automatic growth convenience of array lists requires a more complicated syntax for accessing the elements. The reason is that the ArrayList class is not a part of the Java programming language; it is just a utility class programmed by someone and supplied in the standard library. Instead of the pleasant [] syntax to access or change the element of an array, you use the get and set methods. For example, to set the ith element, use staff.set(i, harry); This is equivalent to a[i] = harry; for an array a. (As with arrays, the index values are zero-based.) From the Library of Hristo Dimov Hristov 252 Chapter 5 Inheritance CAUTION: Do not call list.set(i, x) until the size of the array list is larger than i. For example, the following code is wrong: var list = new ArrayList<Employee>(100); // capacity 100, size 0 list.set(0, x); // no element 0 yet Use the add method instead of set to fill up an array, and use set only to replace a previously added element. To get an array list element, use Employee e = staff.get(i); This is equivalent to Employee e = a[i]; NOTE: When there were no generic classes, the get method of the raw ArrayList class had no choice but to return an Object. Consequently, callers of get had to cast the returned value to the desired type: Employee e = (Employee) staff.get(i); The raw ArrayList is also a bit dangerous. Its add and set methods accept objects of any type. A call staff.set(i, "Harry Hacker"); compiles without so much as a warning, and you run into grief only when you retrieve the object and try to cast it. If you use an ArrayList<Employee> instead, the compiler will detect this error. You can sometimes get the best of both worlds—flexible growth and convenient element access—with the following trick. First, make an array list and add all the elements: var list = new ArrayList<X>(); while (. . .) { x = . . .; list.add(x); } When you are done, use the toArray method to copy the elements into an array: var a = new X[list.size()]; list.toArray(a); From the Library of Hristo Dimov Hristov 5.3 Generic Array Lists 253 Sometimes, you need to add elements in the middle of an array list. Use the add method with an index parameter: int n = staff.size() / 2; staff.add(n, e); The elements at locations n and above are shifted up to make room for the new entry. If the new size of the array list after the insertion exceeds the capacity, the array list reallocates its storage array. Similarly, you can remove an element from the middle of an array list: Employee e = staff.remove(n); The elements located above it are copied down, and the size of the array is reduced by one. Inserting and removing elements is not terribly efficient. It is probably not worth worrying about for small array lists. But if you store many elements and frequently insert and remove in the middle of a collection, consider using a linked list instead. We explain how to program with linked lists in Chapter 9. You can use the “for each” loop to traverse the contents of an array list: for (Employee e : staff) do something with e This loop has the same effect as for (int i = 0; i < staff.size(); i++) { Employee e = staff.get(i); do something with e } Listing 5.11 is a modification of the EmployeeTest program of Chapter 4. The array is replaced by an ArrayList<Employee>. Note the following changes: Employee[] • • • • You You You You don’t have to specify the array size. use add to add as many elements as you like. use size() instead of length to count the number of elements. use a.get(i) instead of a[i] to access an element. Listing 5.11 arrayList/ArrayListTest.java 1 package arrayList; 2 3 import java.util.*; (Continues) From the Library of Hristo Dimov Hristov 254 Chapter 5 Inheritance Listing 5.11 (Continued) 4 5 6 7 8 9 10 11 12 13 14 15 /** * This program demonstrates the ArrayList class. * @version 1.11 2012-01-26 * @author Cay Horstmann */ public class ArrayListTest { public static void main(String[] args) { // fill the staff array list with three Employee objects var staff = new ArrayList<Employee>(); 16 staff.add(new Employee("Carl Cracker", 75000, 1987, 12, 15)); staff.add(new Employee("Harry Hacker", 50000, 1989, 10, 1)); staff.add(new Employee("Tony Tester", 40000, 1990, 3, 15)); 17 18 19 20 // raise everyone's salary by 5% for (Employee e : staff) e.raiseSalary(5); 21 22 23 24 // print out information about all Employee objects for (Employee e : staff) System.out.println("name=" + e.getName() + ",salary=" + e.getSalary() + ",hireDay=" + e.getHireDay()); 25 26 27 28 } 29 30 } java.util.ArrayList<E> 1.2 • E set(int index, E obj) puts the value obj in the array list at the specified index, returning the previous contents. • E get(int index) gets the value stored at a specified index. • void add(int index, E obj) shifts up elements to insert obj at the specified index. • E remove(int index) removes the element at the given index and shifts down all elements above it. The removed element is returned. From the Library of Hristo Dimov Hristov 5.3 Generic Array Lists 255 5.3.3 Compatibility between Typed and Raw Array Lists In your own code, you will always want to use type parameters for added safety. In this section, you will see how to interoperate with legacy code that does not use type parameters. Suppose you have the following legacy class: public class EmployeeDB { public void update(ArrayList list) { . . . } public ArrayList find(String query) { . . . } } You can pass a typed array list to the update method without any casts. ArrayList<Employee> staff = . . .; employeeDB.update(staff); The staff object is simply passed to the update method. CAUTION: Even though you get no error or warning from the compiler, this call is not completely safe. The update method might add elements into the array list that are not of type Employee. When these elements are retrieved, an exception occurs. This sounds scary, but if you think about it, the behavior is simply as it was before generics were added to Java. The integrity of the virtual machine is never jeopardized. In this situation, you do not lose security, but you also do not benefit from the compile-time checks. Conversely, when you assign a raw ArrayList to a typed one, you get a warning. ArrayList<Employee> result = employeeDB.find(query); // yields warning NOTE: To see the text of the warning, compile with the option -Xlint:unchecked. Using a cast does not make the warning go away. ArrayList<Employee> result = (ArrayList<Employee>) employeeDB.find(query); // yields another warning Instead, you get a different warning, telling you that the cast is misleading. This is the consequence of a somewhat unfortunate limitation of generic types in Java. For compatibility, the compiler translates all typed array lists into raw ArrayList objects after checking that the type rules were not violated. In a From the Library of Hristo Dimov Hristov 256 Chapter 5 Inheritance running program, all array lists are the same—there are no type parameters in the virtual machine. Thus, the casts (ArrayList) and (ArrayList<Employee>) carry out identical runtime checks. There isn’t much you can do about that situation. When you interact with legacy code, study the compiler warnings and satisfy yourself that the warnings are not serious. Once you are satisfied, you can tag the variable that receives the cast with the @SuppressWarnings("unchecked") annotation, like this: @SuppressWarnings("unchecked") ArrayList<Employee> result = (ArrayList<Employee>) employeeDB.find(query); // yields another warning 5.4 Object Wrappers and Autoboxing Occasionally, you need to convert a primitive type like int to an object. All primitive types have class counterparts. For example, a class Integer corresponds to the primitive type int. These kinds of classes are usually called wrappers. The wrapper classes have obvious names: Integer, Long, Float, Double, Short, Byte, Character, and Boolean. (The first six inherit from the common superclass Number.) The wrapper classes are immutable—you cannot change a wrapped value after the wrapper has been constructed. They are also final, so you cannot subclass them. Suppose we want an array list of integers. Unfortunately, the type parameter inside the angle brackets cannot be a primitive type. It is not possible to form an ArrayList<int>. Here, the Integer wrapper class comes in. It is OK to declare an array list of Integer objects. var list = new ArrayList<Integer>(); CAUTION: An ArrayList<Integer> is far less efficient than an int[] array because each value is separately wrapped inside an object. You would only want to use this construct for small collections when programmer convenience is more important than efficiency. Fortunately, there is a useful feature that makes it easy to add an element of type int to an ArrayList<Integer>. The call list.add(3); is automatically translated to From the Library of Hristo Dimov Hristov 5.4 Object Wrappers and Autoboxing 257 list.add(Integer.valueOf(3)); This conversion is called autoboxing. NOTE: You might think that autowrapping would be more consistent, but the “boxing” metaphor was taken from C#. Conversely, when you assign an Integer object to an int value, it is automatically unboxed. That is, the compiler translates int n = list.get(i); into int n = list.get(i).intValue(); Automatic boxing and unboxing even works with arithmetic expressions. For example, you can apply the increment operator to a wrapper reference: Integer n = 3; n++; The compiler automatically inserts instructions to unbox the object, increment the resulting value, and box it back. In most cases, you get the illusion that the primitive types and their wrappers are one and the same. There is just one point in which they differ considerably: identity. As you know, the == operator, applied to wrapper objects, only tests whether the objects have identical memory locations. The following comparison would therefore probably fail: Integer a = 1000; Integer b = 1000; if (a == b) . . . However, a Java implementation may, if it chooses, wrap commonly occurring values into identical objects, and thus the comparison might succeed. This ambiguity is not what you want. The remedy is to call the equals method when comparing wrapper objects. NOTE: The autoboxing specification requires that boolean, byte, char <= 127, short, and int between -128 and 127 are wrapped into fixed objects. For example, if a and b had been initialized with 100 in the preceding example, then the comparison would have had to succeed. From the Library of Hristo Dimov Hristov 258 Chapter 5 Inheritance There are a couple of other subtleties about autoboxing. First off, since wrapper class references can be null, it is possible for autounboxing to throw a NullPointerException: Integer n = null; System.out.println(2 * n); // throws NullPointerException Also, if you mix Integer and Double types in a conditional expression, then the Integer value is unboxed, promoted to double, and boxed into a Double: Integer n = 1; Double x = 2.0; System.out.println(true ? n : x); // prints 1.0 Finally, let us emphasize that boxing and unboxing is a courtesy of the compiler, not the virtual machine. The compiler inserts the necessary calls when it generates the bytecodes of a class. The virtual machine simply executes those bytecodes. You will often see the number wrappers for another reason. The designers of Java found the wrappers a convenient place to put certain basic methods, such as those for converting strings of digits to numbers. To convert a string to an integer, use the following statement: int x = Integer.parseInt(s); This has nothing to do with Integer objects—parseInt is a static method. But the Integer class was a good place to put it. The API notes show some of the more important methods of the Integer class. The other number classes implement corresponding methods. CAUTION: Some people think that the wrapper classes can be used to implement methods that can modify numeric parameters. However, that is not correct. Recall from Chapter 4 that it is impossible to write a Java method that increments an integer parameter because parameters to Java methods are always passed by value. public static void triple(int x) // won't work { x = 3 * x; // modifies local variable } Could we overcome this by using an Integer instead of an int? From the Library of Hristo Dimov Hristov 5.4 Object Wrappers and Autoboxing 259 public static void triple(Integer x) // won't work { . . . } The problem is that Integer objects are immutable: The information contained inside the wrapper can’t change. You cannot use these wrapper classes to create a method that modifies numeric parameters. If you really want to write a method to change numeric parameters, you can use one of the holder types defined in the org.omg.CORBA package: IntHolder, BooleanHolder, and so on. Each holder type has a public (!) field value through which you can access the stored value. public static void triple(IntHolder x) { x.value = 3 * x.value; } java.lang.Integer 1.0 • int intValue() returns the value of this Integer object as an int (overrides the intValue method in the Number class). • static String toString(int i) returns a new String object representing the number i in base 10. • static String toString(int i, int radix) lets you return a representation of the number i in the base specified by the radix parameter. • static int parseInt(String s) • static int parseInt(String s, int radix) returns the integer whose digits are contained in the string s. The string must represent an integer in base 10 (for the first method) or in the base given by the radix parameter (for the second method). • static Integer valueOf(String s) • static Integer valueOf(String s, int radix) returns a new Integer object initialized to the integer whose digits are contained in the string s. The string must represent an integer in base 10 (for the first method) or in the base given by the radix parameter (for the second method). From the Library of Hristo Dimov Hristov 260 Chapter 5 Inheritance java.text.NumberFormat 1.1 • Number parse(String s) returns the numeric value, assuming the specified String represents a number. 5.5 Methods with a Variable Number of Parameters It is possible to provide methods that can be called with a variable number of parameters. (These are sometimes called “varargs” methods.) You have already seen such a method: printf. For example, the calls System.out.printf("%d", n); and System.out.printf("%d %s", n, "widgets"); both call the same method, even though one call has two parameters and the other has three. The printf method is defined like this: public class PrintStream { public PrintStream printf(String fmt, Object... args) { return format(fmt, args); } } Here, the ellipsis ... is part of the Java code. It denotes that the method can receive an arbitrary number of objects (in addition to the fmt parameter). The printf method actually receives two parameters: the format string and an Object[] array that holds all other parameters. (If the caller supplies integers or other primitive type values, autoboxing turns them into objects.) It now faces the unenviable task of scanning the fmt string and matching up the ith format specifier with the value args[i]. In other words, for the implementor of printf, the Object... parameter type is exactly the same as Object[]. The compiler needs to transform each call to printf, bundling the parameters into an array and autoboxing as necessary: System.out.printf("%d %s", new Object[] { new Integer(n), "widgets" } ); You can define your own methods with variable parameters, and you can specify any type for the parameters, even a primitive type. Here is a simple From the Library of Hristo Dimov Hristov 5.6 Enumeration Classes 261 example: a function that computes the maximum of a variable number of values. public static double max(double... values) { double largest = Double.NEGATIVE_INFINITY; for (double v : values) if (v > largest) largest = v; return largest; } Simply call the function like this: double m = max(3.1, 40.4, -5); The compiler passes a new double[] { 3.1, 40.4, -5 } to the max function. NOTE: It is legal to pass an array as the last parameter of a method with variable parameters. For example: System.out.printf("%d %s", new Object[] { new Integer(1), "widgets" } ); Therefore, you can redefine an existing function whose last parameter is an array to a method with variable parameters, without breaking any existing code. For example, MessageFormat.format was enhanced in this way in Java 5. If you like, you can even declare the main method as public static void main(String... args) 5.6 Enumeration Classes You saw in Chapter 3 how to define enumerated types. Here is a typical example: public enum Size { SMALL, MEDIUM, LARGE, EXTRA_LARGE } The type defined by this declaration is actually a class. The class has exactly four instances—it is not possible to construct new objects. Therefore, you never need to use equals for values of enumerated types. Simply use == to compare them. You can, if you like, add constructors, methods, and fields to an enumerated type. Of course, the constructors are only invoked when the enumerated constants are constructed. Here is an example: public enum Size { From the Library of Hristo Dimov Hristov 262 Chapter 5 Inheritance SMALL("S"), MEDIUM("M"), LARGE("L"), EXTRA_LARGE("XL"); private String abbreviation; private Size(String abbreviation) { this.abbreviation = abbreviation; } public String getAbbreviation() { return abbreviation; } } The constructor of an enumeration is always private. You can omit the private modifier, as in the preceding example. It is a syntax error to declare an enum constructor as public or protected. All enumerated types are subclasses of the class Enum. They inherit a number of methods from that class. The most useful one is toString, which returns the name of the enumerated constant. For example, Size.SMALL.toString() returns the string "SMALL". The converse of toString is the static valueOf method. For example, the statement Size s = Enum.valueOf(Size.class, "SMALL"); sets s to Size.SMALL. Each enumerated type has a static values method that returns an array of all values of the enumeration. For example, the call Size[] values = Size.values(); returns the array with elements Size.SMALL, Size.MEDIUM, Size.LARGE, and Size.EXTRA_LARGE. The ordinal method yields the position of an enumerated constant in the enum declaration, counting from zero. For example, Size.MEDIUM.ordinal() returns 1. The short program in Listing 5.12 demonstrates how to work with enumerated types. NOTE: The Enum class has a type parameter that we have ignored for simplicity. For example, the enumerated type Size actually extends Enum<Size>. The type parameter is used in the compareTo method. (We discuss the compareTo method in Chapter 6 and type parameters in Chapter 8.) Listing 5.12 enums/EnumTest.java 1 package enums; 2 3 import java.util.*; From the Library of Hristo Dimov Hristov 5.6 Enumeration Classes 263 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 /** * This program demonstrates enumerated types. * @version 1.0 2004-05-24 * @author Cay Horstmann */ public class EnumTest { public static void main(String[] args) { var in = new Scanner(System.in); System.out.print("Enter a size: (SMALL, MEDIUM, LARGE, EXTRA_LARGE) "); String input = in.next().toUpperCase(); Size size = Enum.valueOf(Size.class, input); System.out.println("size=" + size); System.out.println("abbreviation=" + size.getAbbreviation()); if (size == Size.EXTRA_LARGE) System.out.println("Good job--you paid attention to the _."); } } 24 25 26 27 enum Size { SMALL("S"), MEDIUM("M"), LARGE("L"), EXTRA_LARGE("XL"); 28 private Size(String abbreviation) { this.abbreviation = abbreviation; } public String getAbbreviation() { return abbreviation; } 29 30 31 private String abbreviation; 32 33 } java.lang.Enum<E> 5 • static Enum valueOf(Class enumClass, String name) returns the enumerated constant of the given class with the given name. • String toString() returns the name of this enumerated constant. • int ordinal() returns the zero-based position of this enumerated constant in the enum declaration. • int compareTo(E other) returns a negative integer if this enumerated constant comes before other, zero if this == other, and a positive integer otherwise. The ordering of the constants is given by the enum declaration. From the Library of Hristo Dimov Hristov 264 Chapter 5 Inheritance 5.7 Reflection The reflection library gives you a very rich and elaborate toolset to write programs that manipulate Java code dynamically. Using reflection, Java can support user interface builders, object-relational mappers, and many other development tools that dynamically inquire about the capabilities of classes. A program that can analyze the capabilities of classes is called reflective. The reflection mechanism is extremely powerful. As the next sections show, you can use it to • Analyze the capabilities of classes at runtime • Inspect objects at runtime—for example, to write a single toString method that works for all classes • Implement generic array manipulation code • Take advantage of Method objects that work just like function pointers in languages such as C++ Reflection is a powerful and complex mechanism; however, it is of interest mainly to tool builders, not application programmers. If you are interested in programming applications rather than tools for other Java programmers, you can safely skip the remainder of this chapter and return to it later. 5.7.1 The Class Class While your program is running, the Java runtime system always maintains what is called runtime type identification on all objects. This information keeps track of the class to which each object belongs. Runtime type information is used by the virtual machine to select the correct methods to execute. However, you can also access this information by working with a special Java class. The class that holds this information is called, somewhat confusingly, Class. The getClass() method in the Object class returns an instance of Class type. Employee e; . . . Class cl = e.getClass(); Just like an Employee object describes the properties of a particular employee, a Class object describes the properties of a particular class. Probably the most commonly used method of Class is getName. This returns the name of the class. For example, the statement System.out.println(e.getClass().getName() + " " + e.getName()); From the Library of Hristo Dimov Hristov 5.7 Reflection 265 prints Employee Harry Hacker if e is an employee, or Manager Harry Hacker if e is a manager. If the class is in a package, the package name is part of the class name: var generator = new Random(); Class cl = generator.getClass(); String name = cl.getName(); // name is set to "java.util.Random" You can obtain a Class object corresponding to a class name by using the static forName method. String className = "java.util.Random"; Class cl = Class.forName(className); Use this method if the class name is stored in a string that varies at runtime. This works if className is the name of a class or interface. Otherwise, the forName method throws a checked exception. See Section 5.7.2, “A Primer on Declaring Exceptions,” on p. 267 for how to supply an exception handler whenever you use this method. TIP: At startup, the class containing your main method is loaded. It loads all classes that it needs. Each of those loaded classes loads the classes that it needs, and so on. That can take a long time for a big application, frustrating the user. You can give the users of your program an illusion of a faster start with the following trick. Make sure the class containing the main method does not explicitly refer to other classes. In it, display a splash screen. Then manually force the loading of other classes by calling Class.forName. A third method for obtaining an object of type Class is a convenient shorthand. If T is any Java type (or the void keyword), then T.class is the matching class object. For example: Class cl1 = Random.class; // if you import java.util.*; Class cl2 = int.class; Class cl3 = Double[].class; Note that a Class object really describes a type, which may or may not be a class. For example, int is not a class, but int.class is nevertheless an object of type Class. From the Library of Hristo Dimov Hristov 266 Chapter 5 Inheritance NOTE: The Class class is actually a generic class. For example, Employee.class is of type Class<Employee>. We are not dwelling on this issue because it would further complicate an already abstract concept. For most practical purposes, you can ignore the type parameter and work with the raw Class type. See Chapter 8 for more information on this issue. CAUTION: For historical reasons, the getName method returns somewhat strange names for array types: • Double[].class.getName() returns "[Ljava.lang.Double;". • int[].class.getName() returns "[I". The virtual machine manages a unique Class object for each type. Therefore, you can use the == operator to compare class objects. For example: if (e.getClass() == Employee.class) . . . This test passes if e is an instance of Employee. Unlike the condition e instanceof Employee, this test fails if e is an instance of a subclass such as Manager. If you have an object of type Class, you can use it to construct instances of the class. Call the getConstructor method to get an object of type Constructor, then use the newInstance method to construct an instance. For example: var className = "java.util.Random"; // or any other name of a class with // a no-arg constructor Class cl = Class.forName(className); Object obj = cl.getConstructor().newInstance(); If the class doesn’t have a constructor without arguments, the getConstructor method throws an exception. You will see in Section 5.7.7, “Invoking Arbitrary Methods and Constructors,” on p. 286 how to invoke other constructors. NOTE: There is a deprecated Class.toInstance method that also constructs an instance with the no-argument constructor. However, if the constructor throws a checked exception, the exception is rethrown without being checked. This violates the compile-time checking of exceptions. In contrast, Constructor.newInstance wraps any constructor exception into an InvocationTargetException. From the Library of Hristo Dimov Hristov 5.7 Reflection 267 C++ NOTE: The newInstance method corresponds to the idiom of a virtual constructor in C++. However, virtual constructors in C++ are not a language feature but just an idiom that needs to be supported by a specialized library. The Class class is similar to the type_info class in C++, and the getClass method is equivalent to the typeid operator. The Java Class is quite a bit more versatile than type_info, though. The C++ type_info can only reveal a string with the name of the type, not create new objects of that type. java.lang.Class 1.0 • static Class forName(String className) returns the Class object representing the class with name className. • Constructor getConstructor(Class... parameterTypes) 1.1 yields an object describing the constructor with the given parameter types. See Section 5.7.7, “Invoking Arbitrary Methods and Constructors,” on p. 286 for more information on how to supply parameter types. java.lang.reflect.Constructor 1.1 • Object newInstance(Object... params) constructs a new instance of the constructor’s declaring class, passing params to the constructor. See Section 5.7.7, “Invoking Arbitrary Methods and Constructors,” on p. 286 for more information on how to supply parameters. java.lang.Throwable 1.0 • void printStackTrace() prints the Throwable object and the stack trace to the standard error stream. 5.7.2 A Primer on Declaring Exceptions We cover exception handling fully in Chapter 7, but in the meantime you will occasionally encounter methods that threaten to throw exceptions. When an error occurs at runtime, a program can “throw an exception.” Throwing an exception is more flexible than terminating the program because you can provide a handler that “catches” the exception and deals with it. From the Library of Hristo Dimov Hristov 268 Chapter 5 Inheritance If you don’t provide a handler, the program terminates and prints a message to the console, giving the type of the exception. You may have already seen exception reports when you accidentally used a null reference or overstepped the bounds of an array. There are two kinds of exceptions: unchecked exceptions and checked exceptions. With checked exceptions, the compiler checks that you, the programmer, are aware of the exception and are prepared to deal with the consequences. However, many common exceptions, such as bounds errors, or accessing a null reference, are unchecked. The compiler does not expect that you provide a handler—after all, you should spend your mental energy on avoiding these mistakes rather than coding handlers for them. But not all errors are avoidable. If an exception can occur despite your best efforts, then most Java APIs will throw a checked exception. One example is the Class.forName method. There is no way for you to ensure that a class with the given name exists. In Chapter 7, you will see several strategies for exception handling. For now, we just show you the simplest strategy. Whenever a method contains a statement that might throw a checked exception, add a throws clause to the method name. public static void doSomethingWithClass(String name) throws ReflectiveOperationException { Class cl = Class.forName(name); // might throw exception do something with cl } Any method that calls this method also needs a throws declaration. This includes the main method. If an exception actually occurs, the main method terminates with a stack trace. (You will learn in Chapter 7 how to catch exceptions instead of having them terminate your programs.) You only need to supply a throws clause for checked exceptions. It is easy to find out which methods throw checked exceptions—the compiler will complain whenever you call a method that threatens to throw a checked exception and you don’t supply a handler. 5.7.3 Resources Classes often have associated data files, such as: • Image and sound files • Text files with message strings and button labels In Java, such an associated file is called a resource. From the Library of Hristo Dimov Hristov 5.7 Reflection 269 For example, consider a dialog box that displays a message such as the one in Figure 5.3. Figure 5.3 Displaying image and text resources Of course, the book title and copyright year in the panel will change for the next edition of the book. To make it easy to track this change, we will put the text inside a file and not hardcode it as a string. But where should you put a file such as about.txt? Of course, it would be convenient to simply place it with the rest of the program files inside a JAR file. The Class class provides a useful service for locating resource files. Here are the necessary steps: 1. 2. Get the Class object of the class that has a resource—for example, ResourceTest.class. Some methods, such as the getImage method of the ImageIcon class, accept URLs that describe resource locations. Then you call URL url = cl.getResource("about.gif"); 3. Otherwise, use the getResourceAsStream method to obtain an input stream for reading the data in the file. The point is that the Java virtual machine knows how to locate a class, so it can then search for the associated resource in the same location. For example, suppose the ResourceTest class is in a package resources. Then the ResourceTest.class file is located in a resources directory, and you place an icon file into the same directory. Instead of placing a resource file inside the same directory as the class file, you can provide a relative or absolute path such as data/about.txt /corejava/title.txt From the Library of Hristo Dimov Hristov 270 Chapter 5 Inheritance Automating the loading of files is all the resource loading feature does. There are no standard methods for interpreting the contents of resource files. Each program must have its own way of interpreting its resource files. Another common application of resources is the internationalization of programs. Language-dependent strings, such as messages and user interface labels, are stored in resource files, with one file per language. The internationalization API, which is discussed in Chapter 7 of Volume II, supports a standard method for organizing and accessing these localization files. Listing 5.13 is a program that demonstrates resource loading. (Do not worry about the code for reading text and displaying dialogs—we cover those details later.) Compile, build a JAR file, and execute it: javac resource/ResourceTest.java jar cvfe ResourceTest.jar resources.ResourceTest \ resources/*.class resources/*.gif resources/data/*.txt corejava/*.txt java -jar ResourceTest.jar Move the JAR file to a different directory and run it again to check that the program reads the resource files from the JAR file, not from the current directory. Listing 5.13 resources/ResourceTest.java 1 package resources; 2 3 4 5 6 import import import import java.io.*; java.net.*; java.nio.charset.*; javax.swing.*; 7 8 9 10 11 12 13 14 15 16 17 18 /** * @version 1.5 2018-03-15 * @author Cay Horstmann */ public class ResourceTest { public static void main(String[] args) throws IOException { Class cl = ResourceTest.class; URL aboutURL = cl.getResource("about.gif"); var icon = new ImageIcon(aboutURL); 19 20 21 InputStream stream = cl.getResourceAsStream("data/about.txt"); var about = new String(stream.readAllBytes(), "UTF-8"); 22 From the Library of Hristo Dimov Hristov 5.7 Reflection 271 InputStream stream2 = cl.getResourceAsStream("/corejava/title.txt"); var title = new String(stream2.readAllBytes(), StandardCharsets.UTF_8).trim(); 23 24 25 JOptionPane.showMessageDialog(null, about, title, JOptionPane.INFORMATION_MESSAGE, icon); 26 } 27 28 } java.lang.Class 1.0 • URL getResource(String name) 1.1 • InputStream getResourceAsStream(String name) 1.1 finds the resource in the same place as the class and then returns a URL or input stream that you can use for loading the resource. Returns null if the resource isn’t found, so does not throw an exception for an I/O error. 5.7.4 Using Reflection to Analyze the Capabilities of Classes Here is a brief overview of the most important parts of the reflection mechanism for letting you examine the structure of a class. The three classes Field, Method, and Constructor in the java.lang.reflect package describe the fields, methods, and constructors of a class, respectively. All three classes have a method called getName that returns the name of the item. The Field class has a method getType that returns an object, again of type Class, that describes the field type. The Method and Constructor classes have methods to report the types of the parameters, and the Method class also reports the return type. All three of these classes also have a method called getModifiers that returns an integer, with various bits turned on and off, that describes the modifiers used, such as public and static. You can then use the static methods in the Modifier class in the java.lang.reflect package to analyze the integer that getModifiers returns. Use methods like isPublic, isPrivate, or isFinal in the Modifier class to tell whether a method or constructor was public, private, or final. All you have to do is have the appropriate method in the Modifier class work on the integer that getModifiers returns. You can also use the Modifier.toString method to print the modifiers. The getFields, getMethods, and getConstructors methods of the Class class return arrays of the public fields, methods, and constructors that the class supports. This includes public members of superclasses. The getDeclaredFields, getDeclaredMethods, and getDeclaredConstructors methods of the Class class return arrays consisting of all fields, methods, and constructors that are declared in the class. This includes private, package, and protected members, but not members of superclasses. From the Library of Hristo Dimov Hristov 272 Chapter 5 Inheritance Listing 5.14 shows you how to print out all information about a class. The program prompts you for the name of a class and writes out the signatures of all methods and constructors as well as the names of all instance fields of a class. For example, if you enter java.lang.Double the program prints public class java.lang.Double extends java.lang.Number { public java.lang.Double(java.lang.String); public java.lang.Double(double); public public public public public public public public public public public public public public public public public public public public public int hashCode(); int compareTo(java.lang.Object); int compareTo(java.lang.Double); boolean equals(java.lang.Object); java.lang.String toString(); static java.lang.String toString(double); static java.lang.Double valueOf(java.lang.String); static boolean isNaN(double); boolean isNaN(); static boolean isInfinite(double); boolean isInfinite(); byte byteValue(); short shortValue(); int intValue(); long longValue(); float floatValue(); double doubleValue(); static double parseDouble(java.lang.String); static native long doubleToLongBits(double); static native long doubleToRawLongBits(double); static native double longBitsToDouble(long); public static final double POSITIVE_INFINITY; public static final double NEGATIVE_INFINITY; public static final double NaN; public static final double MAX_VALUE; public static final double MIN_VALUE; public static final java.lang.Class TYPE; private double value; private static final long serialVersionUID; } What is remarkable about this program is that it can analyze any class that the Java interpreter can load, not just the classes that were available when the program was compiled. We will use this program in the next chapter to peek inside the inner classes that the Java compiler generates automatically. From the Library of Hristo Dimov Hristov 5.7 Reflection 273 Listing 5.14 reflection/ReflectionTest.java 1 package reflection; 2 3 4 import java.util.*; import java.lang.reflect.*; 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 /** * This program uses reflection to print all features of a class. * @version 1.11 2018-03-16 * @author Cay Horstmann */ public class ReflectionTest { public static void main(String[] args) throws ReflectiveOperationException { // read class name from command line args or user input String name; if (args.length > 0) name = args[0]; else { var in = new Scanner(System.in); System.out.println("Enter class name (e.g. java.util.Date): "); name = in.next(); } 25 // print class name and superclass name (if != Object) Class cl = Class.forName(name); Class supercl = cl.getSuperclass(); String modifiers = Modifier.toString(cl.getModifiers()); if (modifiers.length() > 0) System.out.print(modifiers + " "); System.out.print("class " + name); if (supercl != null && supercl != Object.class) System.out.print(" extends " + supercl.getName()); 26 27 28 29 30 31 32 33 34 System.out.print("\n{\n"); printConstructors(cl); System.out.println(); printMethods(cl); System.out.println(); printFields(cl); System.out.println("}"); 35 36 37 38 39 40 41 42 } 43 44 45 46 47 /** * Prints all constructors of a class * @param cl a class */ (Continues) From the Library of Hristo Dimov Hristov 274 Chapter 5 Inheritance Listing 5.14 (Continued) 48 49 50 public static void printConstructors(Class cl) { Constructor[] constructors = cl.getDeclaredConstructors(); 51 for (Constructor c : constructors) { String name = c.getName(); System.out.print(" "); String modifiers = Modifier.toString(c.getModifiers()); if (modifiers.length() > 0) System.out.print(modifiers + " "); System.out.print(name + "("); 52 53 54 55 56 57 58 59 // print parameter types Class[] paramTypes = c.getParameterTypes(); for (int j = 0; j < paramTypes.length; j++) { if (j > 0) System.out.print(", "); System.out.print(paramTypes[j].getName()); } System.out.println(");"); 60 61 62 63 64 65 66 67 } 68 69 } 70 71 72 73 74 75 76 77 /** * Prints all methods of a class * @param cl a class */ public static void printMethods(Class cl) { Method[] methods = cl.getDeclaredMethods(); 78 79 80 81 82 for (Method m : methods) { Class retType = m.getReturnType(); String name = m.getName(); 83 84 85 86 87 88 System.out.print(" "); // print modifiers, return type and method name String modifiers = Modifier.toString(m.getModifiers()); if (modifiers.length() > 0) System.out.print(modifiers + " "); System.out.print(retType.getName() + " " + name + "("); 89 90 91 // print parameter types Class[] paramTypes = m.getParameterTypes(); From the Library of Hristo Dimov Hristov 5.7 Reflection 275 for (int j = 0; j < paramTypes.length; j++) { if (j > 0) System.out.print(", "); System.out.print(paramTypes[j].getName()); } System.out.println(");"); 92 93 94 95 96 97 } 98 } 99 100 /** * Prints all fields of a class * @param cl a class */ public static void printFields(Class cl) { Field[] fields = cl.getDeclaredFields(); 101 102 103 104 105 106 107 108 for (Field f : fields) { Class type = f.getType(); String name = f.getName(); System.out.print(" "); String modifiers = Modifier.toString(f.getModifiers()); if (modifiers.length() > 0) System.out.print(modifiers + " "); System.out.println(type.getName() + " " + name + ";"); } 109 110 111 112 113 114 115 116 117 } 118 119 } java.lang.Class 1.0 • Field[] getFields() 1.1 • Field[] getDeclaredFields() 1.1 getFields returns an array containing Field objects for the public fields of this class or its superclasses; getDeclaredField returns an array of Field objects for all fields of this class. The methods return an array of length 0 if there are no such fields or if the Class object represents a primitive or array type. • Method[] getMethods() 1.1 • Method[] getDeclaredMethods() 1.1 returns an array containing Method objects: getMethods returns public methods and includes inherited methods; getDeclaredMethods returns all methods of this class or interface but does not include inherited methods. (Continues) From the Library of Hristo Dimov Hristov 276 Chapter 5 Inheritance java.lang.Class 1.0 (Continued) • Constructor[] getConstructors() 1.1 • Constructor[] getDeclaredConstructors() 1.1 returns an array containing Constructor objects that give you all the public constructors (for getConstructors) or all constructors (for getDeclaredConstructors) of the class represented by this Class object. • String getPackageName() 9 gets the name of the package containing this type, or the package of the element type if this type is an array type, or "java.lang" if this type is a primitive type. java.lang.reflect.Field 1.1 java.lang.reflect.Method 1.1 java.lang.reflect.Constructor 1.1 • Class getDeclaringClass() returns the Class object for the class that defines this constructor, method, or field. • Class[] getExceptionTypes() (in Constructor and Method classes) returns an array of Class objects that represent the types of the exceptions thrown by the method. • int getModifiers() returns an integer that describes the modifiers of this constructor, method, or field. Use the methods in the Modifier class to analyze the return value. • String getName() returns a string that is the name of the constructor, method, or field. • Class[] getParameterTypes() (in Constructor and Method classes) returns an array of Class objects that represent the types of the parameters. • Class getReturnType() (in Method class) returns a Class object that represents the return type. java.lang.reflect.Modifier 1.1 • static String toString(int modifiers) returns a string with the modifiers that correspond to the bits set in modifiers. (Continues) From the Library of Hristo Dimov Hristov 5.7 Reflection 277 java.lang.reflect.Modifier 1.1 (Continued) • • • • • • • • • • • static static static static static static static static static static static boolean boolean boolean boolean boolean boolean boolean boolean boolean boolean boolean isAbstract(int modifiers) isFinal(int modifiers) isInterface(int modifiers) isNative(int modifiers) isPrivate(int modifiers) isProtected(int modifiers) isPublic(int modifiers) isStatic(int modifiers) isStrict(int modifiers) isSynchronized(int modifiers) isVolatile(int modifiers) tests the bit in the modifiers value that corresponds to the modifier in the method name. 5.7.5 Using Reflection to Analyze Objects at Runtime In the preceding section, we saw how we can find out the names and types of the data fields of any object: • Get the corresponding Class object. • Call getDeclaredFields on the Class object. In this section, we will go one step further and actually look at the contents of the fields. Of course, it is easy to look at the contents of a specific field of an object whose name and type are known when you write a program. But reflection lets you look at fields of objects that were not known at compile time. The key method to achieve this is the get method in the Field class. If f is an object of type Field (for example, one obtained from getDeclaredFields) and obj is an object of the class of which f is a field, then f.get(obj) returns an object whose value is the current value of the field of obj. This is all a bit abstract, so let’s run through an example. var harry = new Employee("Harry Hacker", 50000, 10, 1, 1989); Class cl = harry.getClass(); // the class object representing Employee Field f = cl.getDeclaredField("name"); // the name field of the Employee class Object v = f.get(harry); // the value of the name field of the harry object, i.e., // the String object "Harry Hacker" From the Library of Hristo Dimov Hristov 278 Chapter 5 Inheritance Of course, you can also set the values that you can get. The call f.set(obj, value) sets the field represented by f of the object obj to the new value. Actually, there is a problem with this code. Since the name field is a private field, the get and set methods will throw an IllegalAccessException. You can only use get and set with accessible fields. The security mechanism of Java lets you find out what fields an object has, but it won’t let you read and write the values of those fields unless you have permission. The default behavior of the reflection mechanism is to respect Java access control. However, you can override access control by invoking the setAccessible method on a Field, Method, or Constructor object. For example: f.setAccessible(true); // now OK to call f.get(harry) The setAccessible method is a method of the AccessibleObject class, the common superclass of the Field, Method, and Constructor classes. This feature is provided for debuggers, persistent storage, and similar mechanisms. We use it for a generic toString method later in this section. The call to setAccessible throws an exception if the access is not granted. The access can be denied by the module system (Chapter 9 of Volume II) or a security manager (Chapter 10 of Volume II). The use of security managers is not common. However, as of Java 9, every program contains modules since the Java API is modularized. Because so many libraries make use of reflection, Java 9 and 10 only give a warning when you use reflection to access a nonpublic feature inside a module. For example, the sample program at the end of this section looks into the internals of ArrayList and Integer objects. When you run the program, the following ominous message appears in the console: WARNING: An illegal reflective access operation has occurred WARNING: Illegal reflective access by objectAnalyzer.ObjectAnalyzer (file:/home/cay /books/cj11/code/v1ch05/bin/) to field java.util.ArrayList.serialVersionUID WARNING: Please consider reporting this to the maintainers of objectAnalyzer.ObjectAnalyzer WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations WARNING: All illegal access operations will be denied in a future release For now, you can deactivate the warning. You need to “open” the java.util and java.lang packages in the java.base module to the “unnamed module.” The details are in Chapter 9 of Volume II. Here is the syntax: java --add-opens java.base/java.util=ALL-UNNAMED \ --add-opens java.base/java.lang=ALL-UNNAMED \ objectAnalyzer.ObjectAnalyzerTest From the Library of Hristo Dimov Hristov 5.7 Reflection 279 Alternatively, you can see how the program will behave in a future version of Java, by running: java --illegal-access=deny objectAnalyzer/ObjectAnalyzerTest Then the program will simply fail with an IllegalAccessException. NOTE: It is possible that future libraries will use variable handles instead of reflection for reading and writing fields. A VarHandle is similar to a Field. You can use it to read or write a specific field of any instance of a specific class. However, to obtain a VarHandle, the library code needs a Lookup object: public Object getFieldValue(Object obj, String fieldName, Lookup lookup) throws NoSuchFieldException, IllegalAccessException { Class<?> cl = obj.getClass(); Field field = cl.getDeclaredField(fieldName); VarHandle handle = MethodHandles.privateLookupIn(cl, lookup) .unreflectVarHandle(field); return handle.get(obj); } This works provided the Lookup object is generated in the module that has the permission to access the field. Some method in the module simply calls MethodHandles.lookup(), which yields an object encapsulating the access rights of the caller. In this way, one module can give permission for accessing private members to another module. The practical issue is how those permissions can be given with a minimum of hassle. While we can still do so, let us look at a generic toString method that works for any class (see Listing 5.15). The generic toString method uses getDeclaredFields to obtain all data fields and the setAccessible convenience method to make all fields accessible. For each field, it obtains the name and the value. Each value is turned into a string by recursively invoking toString. The generic toString method needs to address a couple of complexities. Cycles of references could cause an infinite recursion. Therefore, the ObjectAnalyzer keeps track of objects that were already visited. Also, to peek inside arrays, you need a different approach. You’ll learn about the details in the next section. You can use this toString method to peek inside any object. For example, the call var squares = new ArrayList<Integer>(); for (int i = 1; i <= 5; i++) squares.add(i * i); System.out.println(new ObjectAnalyzer().toString(squares)); From the Library of Hristo Dimov Hristov 280 Chapter 5 Inheritance yields the printout java.util.ArrayList[elementData=class java.lang.Object[]{java.lang.Integer[value=1][][], java.lang.Integer[value=4][][],java.lang.Integer[value=9][][], java.lang.Integer[value=16][][], java.lang.Integer[value=25][][],null,null,null,null,null},size=5][modCount=5][][] You can use this generic toString method to implement the toString methods of your own classes, like this: public String toString() { return new ObjectAnalyzer().toString(this); } This is a hassle-free and undoubtedly useful method for supplying a universal toString method. However, before you get too excited about never having to implement toString again, remember that the days of uncontrolled access to internals are numbered. Listing 5.15 objectAnalyzer/ObjectAnalyzerTest.java 1 package objectAnalyzer; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 /** * This program uses reflection to spy on objects. * @version 1.13 2018-03-16 * @author Cay Horstmann */ public class ObjectAnalyzerTest { public static void main(String[] args) throws ReflectiveOperationException { var squares = new ArrayList<Integer>(); for (int i = 1; i <= 5; i++) squares.add(i * i); System.out.println(new ObjectAnalyzer().toString(squares)); } } Listing 5.16 objectAnalyzer/ObjectAnalyzer.java 1 package objectAnalyzer; 2 3 4 import java.lang.reflect.AccessibleObject; import java.lang.reflect.Array; From the Library of Hristo Dimov Hristov 5.7 5 6 7 Reflection 281 import java.lang.reflect.Field; import java.lang.reflect.Modifier; import java.util.ArrayList; 8 9 10 11 public class ObjectAnalyzer { private ArrayList<Object> visited = new ArrayList<>(); 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 /** * Converts an object to a string representation that lists all fields. * @param obj an object * @return a string with the object's class name and all field names and values */ public String toString(Object obj) throws ReflectiveOperationException { if (obj == null) return "null"; if (visited.contains(obj)) return "..."; visited.add(obj); Class cl = obj.getClass(); if (cl == String.class) return (String) obj; if (cl.isArray()) { String r = cl.getComponentType() + "[]{"; for (int i = 0; i < Array.getLength(obj); i++) { if (i > 0) r += ","; Object val = Array.get(obj, i); if (cl.getComponentType().isPrimitive()) r += val; else r += toString(val); } return r + "}"; } 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 String r = cl.getName(); // inspect the fields of this class and all superclasses do { r += "["; Field[] fields = cl.getDeclaredFields(); AccessibleObject.setAccessible(fields, true); // get the names and values of all fields for (Field f : fields) { if (!Modifier.isStatic(f.getModifiers())) { if (!r.endsWith("[")) r += ","; r += f.getName() + "="; (Continues) From the Library of Hristo Dimov Hristov 282 Chapter 5 Inheritance Listing 5.16 (Continued) Class t = f.getType(); Object val = f.get(obj); if (t.isPrimitive()) r += val; else r += toString(val); 53 54 55 56 } } r += "]"; cl = cl.getSuperclass(); 57 58 59 60 } while (cl != null); 61 62 63 return r; 64 } 65 66 } java.lang.reflect.AccessibleObject 1.2 • void setAccessible(boolean flag) sets or clears the accessibility flag for this accessible object, or throws an IllegalAccessException if the access is denied. • boolean trySetAccessible() 9 sets the accessibility flag for this accessible object, or returns false if the access is denied. • boolean isAccessible() gets the value of the accessibility flag for this accessible object. • static void setAccessible(AccessibleObject[] array, boolean flag) is a convenience method to set the accessibility flag for an array of objects. java.lang.Class 1.1 • Field getField(String name) • Field[] getFields() gets the public field with the given name, or an array of all fields. • Field getDeclaredField(String name) • Field[] getDeclaredFields() gets the field that is declared in this class with the given name, or an array of all fields. From the Library of Hristo Dimov Hristov 5.7 Reflection 283 java.lang.reflect.Field 1.1 • Object get(Object obj) gets the value of the field described by this Field object in the object obj. • void set(Object obj, Object newValue) sets the field described by this Field object in the object obj to a new value. 5.7.6 Using Reflection to Write Generic Array Code The Array class in the java.lang.reflect package allows you to create arrays dynamically. This is used, for example, in the implementation of the copyOf method in the Arrays class. Recall how this method can be used to grow an array that has become full. var a = new Employee[100]; . . . // array is full a = Arrays.copyOf(a, 2 * a.length); How can one write such a generic method? It helps that an Employee[] array can be converted to an Object[] array. That sounds promising. Here is a first attempt: public static Object[] badCopyOf(Object[] a, int newLength) // not useful { var newArray = new Object[newLength]; System.arraycopy(a, 0, newArray, 0, Math.min(a.length, newLength)); return newArray; } However, there is a problem with actually using the resulting array. The type of array that this code returns is an array of objects (Object[]) because we created the array using the line of code new Object[newLength] An array of objects cannot be cast to an array of employees (Employee[]). The virtual machine would generate a ClassCastException at runtime. The point is that, as we mentioned earlier, a Java array remembers the type of its entries— that is, the element type used in the new expression that created it. It is legal to cast an Employee[] temporarily to an Object[] array and then cast it back, but an array that started its life as an Object[] array can never be cast into an Employee[] array. To write this kind of generic array code, we need to be able to make a new array of the same type as the original array. For this, we need the methods of the Array class in the java.lang.reflect package. The key is the static newInstance method of the Array class that constructs a new array. You From the Library of Hristo Dimov Hristov 284 Chapter 5 Inheritance must supply the type for the entries and the desired length as parameters to this method. Object newArray = Array.newInstance(componentType, newLength); To actually carry this out, we need to get the length and the component type of the new array. We obtain the length by calling Array.getLength(a). The static getLength method of the Array class returns the length of an array. To get the component type of the new array: 1. 2. 3. First, get the class object of a. Confirm that it is indeed an array. Use the getComponentType method of the Class class (which is defined only for class objects that represent arrays) to find the right type for the array. Why is getLength a method of Array but getComponentType a method of Class? We don’t know—the distribution of the reflection methods seems a bit ad hoc at times. Here’s the code: public static Object goodCopyOf(Object a, int newLength) { Class cl = a.getClass(); if (!cl.isArray()) return null; Class componentType = cl.getComponentType(); int length = Array.getLength(a); Object newArray = Array.newInstance(componentType, newLength); System.arraycopy(a, 0, newArray, 0, Math.min(length, newLength)); return newArray; } Note that this copyOf method can be used to grow arrays of any type, not just arrays of objects. int[] a = { 1, 2, 3, 4, 5 }; a = (int[]) goodCopyOf(a, 10); To make this possible, the parameter of goodCopyOf is declared to be of type Object, not an array of objects (Object[]). The integer array type int[] can be converted to an Object, but not to an array of objects! Listing 5.17 shows both methods in action. Note that the cast of the return value of badcopyOf will throw an exception. From the Library of Hristo Dimov Hristov 5.7 Reflection 285 Listing 5.17 arrays/CopyOfTest.java 1 package arrays; 2 3 4 import java.lang.reflect.*; import java.util.*; 5 6 7 8 9 10 11 12 13 14 15 16 17 /** * This program demonstrates the use of reflection for manipulating arrays. * @version 1.2 2012-05-04 * @author Cay Horstmann */ public class CopyOfTest { public static void main(String[] args) { int[] a = { 1, 2, 3 }; a = (int[]) goodCopyOf(a, 10); System.out.println(Arrays.toString(a)); 18 String[] b = { "Tom", "Dick", "Harry" }; b = (String[]) goodCopyOf(b, 10); System.out.println(Arrays.toString(b)); 19 20 21 22 System.out.println("The following call will generate an exception."); b = (String[]) badCopyOf(b, 10); 23 24 25 } 26 27 28 29 30 31 32 33 34 35 36 37 38 39 /** * This method attempts to grow an array by allocating a new array and copying all elements. * @param a the array to grow * @param newLength the new length * @return a larger array that contains all elements of a. However, the returned * array has type Object[], not the same type as a */ public static Object[] badCopyOf(Object[] a, int newLength) // not useful { var newArray = new Object[newLength]; System.arraycopy(a, 0, newArray, 0, Math.min(a.length, newLength)); return newArray; } 40 41 42 43 44 45 /** * This method grows an array by allocating a new array of the same type and * copying all elements. * @param a the array to grow. This can be an object array or a primitive * type array (Continues) From the Library of Hristo Dimov Hristov 286 Chapter 5 Inheritance Listing 5.17 (Continued) * @return a larger array that contains all elements of a. */ public static Object goodCopyOf(Object a, int newLength) { Class cl = a.getClass(); if (!cl.isArray()) return null; Class componentType = cl.getComponentType(); int length = Array.getLength(a); Object newArray = Array.newInstance(componentType, newLength); System.arraycopy(a, 0, newArray, 0, Math.min(length, newLength)); return newArray; } 46 47 48 49 50 51 52 53 54 55 56 57 58 } java.lang.reflect.Array 1.1 • static Object get(Object array, int index) • static xxx getXxx(Object array, int index) (xxx is one of the primitive types boolean, byte, char, double, float, int, long, or short.) These methods return the value of the given array that is stored at the given index. • static void set(Object array, int index, Object newValue) • static setXxx(Object array, int index, xxx newValue) (xxx is one of the primitive types boolean, byte, char, double, float, int, long, or short.) These methods store a new value into the given array at the given index. • static int getLength(Object array) returns the length of the given array. • static Object newInstance(Class componentType, int length) • static Object newInstance(Class componentType, int[] lengths) returns a new array of the given component type with the given dimensions. 5.7.7 Invoking Arbitrary Methods and Constructors In C and C++, you can execute an arbitrary function through a function pointer. On the surface, Java does not have method pointers—that is, ways of giving the location of a method to another method, so that the second method can invoke it later. In fact, the designers of Java have said that method pointers are dangerous and error-prone, and that Java interfaces and lambda expressions (discussed in the next chapter) are a superior solution. However, the reflection mechanism allows you to call arbitrary methods. From the Library of Hristo Dimov Hristov 5.7 Reflection 287 Recall that you can inspect a field of an object with the get method of the Field class. Similarly, the Method class has an invoke method that lets you call the method that is wrapped in the current Method object. The signature for the invoke method is Object invoke(Object obj, Object... args) The first parameter is the implicit parameter, and the remaining objects provide the explicit parameters. For a static method, the first parameter is ignored—you can set it to null. For example, if m1 represents the getName method of the Employee class, the following code shows how you can call it: String n = (String) m1.invoke(harry); If the return type is a primitive type, the invoke method will return the wrapper type instead. For example, suppose that m2 represents the getSalary method of the Employee class. Then, the returned object is actually a Double, and you must cast it accordingly. Use automatic unboxing to turn it into a double: double s = (Double) m2.invoke(harry); How do you obtain a Method object? You can, of course, call getDeclaredMethods and search through the returned array of Method objects until you find the method you want. Or, you can call the getMethod method of the Class class. This is similar to the getField method that takes a string with the field name and returns a Field object. However, there may be several methods with the same name, so you need to be careful that you get the right one. For that reason, you must also supply the parameter types of the desired method. The signature of getMethod is Method getMethod(String name, Class... parameterTypes) For example, here is how you can get method pointers to the getName and raiseSalary methods of the Employee class: Method m1 = Employee.class.getMethod("getName"); Method m2 = Employee.class.getMethod("raiseSalary", double.class); Use a similar approach for invoking arbitrary constructors. Supply the constructor’s parameter types to the Class.getConstructor method, and supply the parameter values to the Constructor.newInstance method: Class cl = Random.class; // or any other class with a constructor that // accepts a long parameter Constructor cons = cl.getConstructor(long.class); Object obj = cons.newInstance(42L); From the Library of Hristo Dimov Hristov 288 Chapter 5 Inheritance Now that you have seen the rules for using Method objects, let’s put them to work. Listing 5.18 is a program that prints a table of values for a mathematical function such as Math.sqrt or Math.sin. The printout looks like this: public static native double java.lang.Math.sqrt(double) 1.0000 | 1.0000 2.0000 | 1.4142 3.0000 | 1.7321 4.0000 | 2.0000 5.0000 | 2.2361 6.0000 | 2.4495 7.0000 | 2.6458 8.0000 | 2.8284 9.0000 | 3.0000 10.0000 | 3.1623 The code for printing a table is, of course, independent of the actual function that is being tabulated. double dx = (to - from) / (n - 1); for (double x = from; x <= to; x += dx) { double y = (Double) f.invoke(null, x); System.out.printf("%10.4f | %10.4f%n", x, y); } Here, f is an object of type Method. The first parameter of invoke is null because we are calling a static method. To tabulate the Math.sqrt function, we set f to Math.class.getMethod("sqrt", double.class) That is the method of the Math class that has the name sqrt and a single parameter of type double. Listing 5.18 shows the complete code of the generic tabulator and a couple of test runs. Listing 5.18 methods/MethodTableTest.java 1 package methods; 2 3 import java.lang.reflect.*; 4 5 6 7 8 9 /** * This program shows how to invoke methods through reflection. * @version 1.2 2012-05-04 * @author Cay Horstmann */ From the Library of Hristo Dimov Hristov 5.7 10 11 12 13 14 15 16 17 Reflection 289 public class MethodTableTest { public static void main(String[] args) throws ReflectiveOperationException { // get method pointers to the square and sqrt methods Method square = MethodTableTest.class.getMethod("square", double.class); Method sqrt = Math.class.getMethod("sqrt", double.class); 18 // print tables of x- and y-values printTable(1, 10, 10, square); printTable(1, 10, 10, sqrt); 19 20 21 } 22 23 /** * Returns the square of a number * @param x a number * @return x squared */ public static double square(double x) { return x * x; } 24 25 26 27 28 29 30 31 32 33 /** * Prints a table with x- and y-values for a method * @param from the lower bound for the x-values * @param to the upper bound for the x-values * @param n the number of rows in the table * @param f a method with a double parameter and double return value */ public static void printTable(double from, double to, int n, Method f) throws ReflectiveOperationException { // print out the method as table header System.out.println(f); 34 35 36 37 38 39 40 41 42 43 44 45 46 double dx = (to - from) / (n - 1); 47 48 for (double x = from; x <= to; x += dx) { double y = (Double) f.invoke(null, x); System.out.printf("%10.4f | %10.4f%n", x, y); } 49 50 51 52 53 } 54 55 } As this example clearly shows, you can do anything with Method objects that you can do with function pointers in C (or delegates in C#). Just as in C, this style of programming is usually quite inconvenient, and always error-prone. From the Library of Hristo Dimov Hristov 290 Chapter 5 Inheritance What happens if you invoke a method with the wrong parameters? The invoke method throws an exception. Also, the parameters and return values of invoke are necessarily of type Object. That means you must cast back and forth a lot. As a result, the compiler is deprived of the chance to check your code, so errors surface only during testing, when they are more tedious to find and fix. Moreover, code that uses reflection to get at method pointers is significantly slower than code that simply calls methods directly. For that reason, we suggest that you use Method objects in your own programs only when absolutely necessary. Using interfaces and, as of Java 8, lambda expressions (the subject of the next chapter) is almost always a better idea. In particular, we echo the developers of Java and suggest not using Method objects for callback functions. Using interfaces for the callbacks leads to code that runs faster and is a lot more maintainable. java.lang.reflect.Method 1.1 • public Object invoke(Object implicitParameter, Object[] explicitParameters) invokes the method described by this object, passing the given parameters and returning the value that the method returns. For static methods, pass null as the implicit parameter. Pass primitive type values by using wrappers. Primitive type return values must be unwrapped. 5.8 Design Hints for Inheritance We want to end this chapter with some hints that we have found useful when using inheritance. 1. Place common operations and fields in the superclass. This is why we put the name field into the Person class instead of replicating it in the Employee and Student classes. 2. Don’t use protected fields. Some programmers think it is a good idea to define most instance fields as protected, “just in case,” so that subclasses can access these fields if they need to. However, the protected mechanism doesn’t give much protection, From the Library of Hristo Dimov Hristov 5.8 Design Hints for Inheritance 291 for two reasons. First, the set of subclasses is unbounded—anyone can form a subclass of your classes and then write code that directly accesses protected instance fields, thereby breaking encapsulation. And second, in Java, all classes in the same package have access to protected fields, whether or not they are subclasses. However, protected methods can be useful to indicate methods that are not ready for general use and should be redefined in subclasses. 3. Use inheritance to model the “is–a” relationship. Inheritance is a handy code-saver, but sometimes people overuse it. For example, suppose we need a Contractor class. Contractors have names and hire dates, but they do not have salaries. Instead, they are paid by the hour, and they do not stay around long enough to get a raise. There is the temptation to form a subclass Contractor from Employee and add an hourlyWage field. public class Contractor extends Employee { private double hourlyWage; . . . } This is not a good idea, however, because now each contractor object has both a salary and hourly wage field. It will cause you no end of grief when you implement methods for printing paychecks or tax forms. You will end up writing more code than you would have written by not inheriting in the first place. The contractor-employee relationship fails the “is–a” test. A contractor is not a special case of an employee. 4. Don’t use inheritance unless all inherited methods make sense. Suppose we want to write a Holiday class. Surely every holiday is a day, and days can be expressed as instances of the GregorianCalendar class, so we can use inheritance. class Holiday extends GregorianCalendar { . . . } Unfortunately, the set of holidays is not closed under the inherited operations. One of the public methods of GregorianCalendar is add. And add can turn holidays into nonholidays: Holiday christmas; christmas.add(Calendar.DAY_OF_MONTH, 12); From the Library of Hristo Dimov Hristov 292 Chapter 5 Inheritance Therefore, inheritance is not appropriate in this example. Note that this problem does not arise if you extend LocalDate. Because that class is immutable, there is no method that could turn a holiday into a nonholiday. 5. Don’t change the expected behavior when you override a method. The substitution principle applies not just to syntax but, more importantly, to behavior. When you override a method, you should not unreasonably change its behavior. The compiler can’t help you—it cannot check whether your redefinitions make sense. For example, you can “fix” the issue of the add method in the Holiday class by redefining add, perhaps to do nothing, or to throw an exception, or to move on to the next holiday. However, such a fix violates the substitution principle. The sequence of statements int d1 = x.get(Calendar.DAY_OF_MONTH); x.add(Calendar.DAY_OF_MONTH, 1); int d2 = x.get(Calendar.DAY_OF_MONTH); System.out.println(d2 - d1); should have the expected behavior, no matter whether x is of type GregorianCalendar or Holiday. Of course, therein lies the rub. Reasonable and unreasonable people can argue at length about what the expected behavior is. For example, some authors argue that the substitution principle requires Manager.equals to ignore the bonus field because Employee.equals ignores it. These discussions are pointless if they occur in a vacuum. Ultimately, what matters is that you do not circumvent the intent of the original design when you override methods in subclasses. 6. Use polymorphism, not type information. Whenever you find code of the form if (x is of type 1) action1(x); else if (x is of type 2) action2(x); think polymorphism. Do action1 and action2 represent a common concept? If so, make the concept a method of a common superclass or interface of both types. Then, you can simply call x.action(); From the Library of Hristo Dimov Hristov 5.8 Design Hints for Inheritance 293 and have the dynamic dispatch mechanism inherent in polymorphism launch the correct action. Code that uses polymorphic methods or interface implementations is much easier to maintain and extend than code using multiple type tests. 7. Don’t overuse reflection. The reflection mechanism lets you write programs with amazing generality, by detecting fields and methods at runtime. This capability can be extremely useful for systems programming, but it is usually not appropriate in applications. Reflection is fragile—with it, the compiler cannot help you find programming errors. Any errors are found at runtime and result in exceptions. You have now seen how Java supports the fundamentals of object-oriented programming: classes, inheritance, and polymorphism. In the next chapter, we will tackle two advanced topics that are very important for using Java effectively: interfaces and lambda expressions. From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov CHAPTER 6 Interfaces, Lambda Expressions, and Inner Classes In this chapter • 6.1 Interfaces, page 296 • 6.2 Lambda Expressions, page 322 • 6.3 Inner Classes, page 340 • 6.4 Service Loaders, page 360 • 6.5 Proxies, page 362 You have now learned about classes and inheritance, the key concepts of object-oriented programming in Java. This chapter shows you several advanced techniques that are commonly used. Despite their less obvious nature, you will need to master them to complete your Java tool chest. The first technique, called interfaces, is a way of describing what classes should do, without specifying how they should do it. A class can implement one or more interfaces. You can then use objects of these implementing classes whenever conformance to the interface is required. After we cover interfaces, 295 From the Library of Hristo Dimov Hristov 296 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes we move on to lambda expressions, a concise way to create blocks of code that can be executed at a later point in time. Using lambda expressions, you can express code that uses callbacks or variable behavior in an elegant and concise fashion. We then discuss the mechanism of inner classes. Inner classes are technically somewhat complex—they are defined inside other classes, and their methods can access the fields of the surrounding class. Inner classes are useful when you design collections of cooperating classes. This chapter concludes with a discussion of proxies, objects that implement arbitrary interfaces. A proxy is a very specialized construct that is useful for building system-level tools. You can safely skip that section on first reading. 6.1 Interfaces In the following sections, you will learn what Java interfaces are and how to use them. You will also find out how interfaces have been made more powerful in recent versions of Java. 6.1.1 The Interface Concept In the Java programming language, an interface is not a class but a set of requirements for the classes that want to conform to the interface. Typically, the supplier of some service states: “If your class conforms to a particular interface, then I’ll perform the service.” Let’s look at a concrete example. The sort method of the Arrays class promises to sort an array of objects, but under one condition: The objects must belong to classes that implement the Comparable interface. Here is what the Comparable interface looks like: public interface Comparable { int compareTo(Object other); } This means that any class that implements the Comparable interface is required to have a compareTo method, and the method must take an Object parameter and return an integer. From the Library of Hristo Dimov Hristov 6.1 Interfaces 297 NOTE: As of Java 5, the Comparable interface has been enhanced to be a generic type. public interface Comparable<T> { int compareTo(T other); // parameter has type T } For example, a class that implements Comparable<Employee> must supply a method int compareTo(Employee other) You can still use the “raw” Comparable type without a type parameter. Then the compareTo method has a parameter of type Object, and you have to manually cast that parameter of the compareTo method to the desired type. We will do just that for a little while so that you don’t have to worry about two new concepts at the same time. All methods of an interface are automatically public. For that reason, it is not necessary to supply the keyword public when declaring a method in an interface. Of course, there is an additional requirement that the interface cannot spell out: When calling x.compareTo(y), the compareTo method must actually be able to compare the two objects and return an indication whether x or y is larger. The method is supposed to return a negative number if x is smaller than y, zero if they are equal, and a positive number otherwise. This particular interface has a single method. Some interfaces have multiple methods. As you will see later, interfaces can also define constants. What is more important, however, is what interfaces cannot supply. Interfaces never have instance fields. Before Java 8, methods were never implemented in interfaces. (As you will see in Section 6.1.4, “Static and Private Methods,” on p. 306 and Section 6.1.5, “Default Methods,” on p. 307, it is now possible to supply simple methods in interfaces. Of course, those methods cannot refer to instance fields—interfaces don’t have any.) Supplying instance fields and methods that operate on them is the job of the classes that implement the interface. You can think of an interface as an abstract class with no instance fields. However, there are some differences between these two concepts—we look at them later in some detail. Now, suppose we want to use the sort method of the Arrays class to sort an array of Employee objects. Then the Employee class must implement the Comparable interface. From the Library of Hristo Dimov Hristov 298 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes To make a class implement an interface, you carry out two steps: 1. 2. You declare that your class intends to implement the given interface. You supply definitions for all methods in the interface. To declare that a class implements an interface, use the implements keyword: class Employee implements Comparable Of course, now the Employee class needs to supply the compareTo method. Let’s suppose that we want to compare employees by their salary. Here is an implementation of the compareTo method: public int compareTo(Object otherObject) { Employee other = (Employee) otherObject; return Double.compare(salary, other.salary); } Here, we use the static Double.compare method that returns a negative if the first argument is less than the second argument, 0 if they are equal, and a positive value otherwise. CAUTION: In the interface declaration, the compareTo method was not declared public because all methods in an interface are automatically public. However, when implementing the interface, you must declare the method as public. Otherwise, the compiler assumes that the method has package access—the default for a class. The compiler then complains that you’re trying to supply a more restrictive access privilege. We can do a little better by supplying a type parameter for the generic Comparable interface: class Employee implements Comparable<Employee> { public int compareTo(Employee other) { return Double.compare(salary, other.salary); } . . . } Note that the unsightly cast of the Object parameter has gone away. From the Library of Hristo Dimov Hristov 6.1 Interfaces 299 TIP: The compareTo method of the Comparable interface returns an integer. If the objects are not equal, it does not matter what negative or positive value you return. This flexibility can be useful when you are comparing integer fields. For example, suppose each employee has a unique integer id and you want to sort by the employee ID number. Then you can simply return id - other.id. That value will be some negative value if the first ID number is less than the other, 0 if they are the same ID, and some positive value otherwise. However, there is one caveat: The range of the integers must be small enough so that the subtraction does not overflow. If you know that the IDs are not negative or that their absolute value is at most (Integer.MAX_VALUE - 1) / 2, you are safe. Otherwise, call the static Integer.compare method. Of course, the subtraction trick doesn’t work for floating-point numbers. The difference salary - other.salary can round to 0 if the salaries are close together but not identical. The call Double.compare(x, y) simply returns -1 if x < y or 1 if x > y. NOTE: The documentation of the Comparable interface suggests that the compareTo method should be compatible with the equals method. That is, x.compareTo(y) should be zero exactly when x.equals(y). Most classes in the Java API that implement Comparable follow this advice. A notable exception is BigDecimal. Consider x = new BigDecimal("1.0") and y = new BigDecimal("1.00"). Then x.equals(y) is false because the numbers differ in precision. But x.compareTo(y) is zero. Ideally, it shouldn’t be, but there was no obvious way of deciding which one should come first. Now you saw what a class must do to avail itself of the sorting service—it must implement a compareTo method. That’s eminently reasonable. There needs to be some way for the sort method to compare objects. But why can’t the Employee class simply provide a compareTo method without implementing the Comparable interface? The reason for interfaces is that the Java programming language is strongly typed. When making a method call, the compiler needs to be able to check that the method actually exists. Somewhere in the sort method will be statements like this: if (a[i].compareTo(a[j]) > 0) { // rearrange a[i] and a[j] . . . } From the Library of Hristo Dimov Hristov 300 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes The compiler must know that a[i] actually has a compareTo method. If a is an array of Comparable objects, then the existence of the method is assured because every class that implements the Comparable interface must supply the method. NOTE: You would expect that the sort method in the Arrays class is defined to accept a Comparable[] array so that the compiler can complain if anyone ever calls sort with an array whose element type doesn’t implement the Comparable interface. Sadly, that is not the case. Instead, the sort method accepts an Object[] array and uses a clumsy cast: // approach used in the standard library--not recommended if (((Comparable) a[i]).compareTo(a[j]) > 0) { // rearrange a[i] and a[j] . . . } If a[i] does not belong to a class that implements the Comparable interface, the virtual machine throws an exception. Listing 6.1 presents the full code for sorting an array of instances of the class (Listing 6.2). Employee Listing 6.1 interfaces/EmployeeSortTest.java 1 package interfaces; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 /** * This program demonstrates the use of the Comparable interface. * @version 1.30 2004-02-27 * @author Cay Horstmann */ public class EmployeeSortTest { public static void main(String[] args) { var staff = new Employee[3]; 15 16 17 18 staff[0] = new Employee("Harry Hacker", 35000); staff[1] = new Employee("Carl Cracker", 75000); staff[2] = new Employee("Tony Tester", 38000); 19 20 Arrays.sort(staff); 21 From the Library of Hristo Dimov Hristov 6.1 301 // print out information about all Employee objects for (Employee e : staff) System.out.println("name=" + e.getName() + ",salary=" + e.getSalary()); 22 23 24 } 25 26 Interfaces } Listing 6.2 interfaces/Employee.java 1 package interfaces; 2 3 4 5 6 public class Employee implements Comparable<Employee> { private String name; private double salary; 7 public Employee(String name, double salary) { this.name = name; this.salary = salary; } 8 9 10 11 12 13 public String getName() { return name; } 14 15 16 17 18 public double getSalary() { return salary; } 19 20 21 22 23 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 24 25 26 27 28 29 /** * Compares employees by salary * @param other another Employee object * @return a negative value if this employee has a lower salary than * otherObject, 0 if the salaries are the same, a positive value otherwise */ public int compareTo(Employee other) { return Double.compare(salary, other.salary); } 30 31 32 33 34 35 36 37 38 39 40 } From the Library of Hristo Dimov Hristov 302 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes java.lang.Comparable<T> 1.0 • int compareTo(T other) compares this object with other and returns a negative integer if this object is less than other, zero if they are equal, and a positive integer otherwise. java.util.Arrays 1.2 • static void sort(Object[] a) sorts the elements in the array a. All elements in the array must belong to classes that implement the Comparable interface, and they must all be comparable to each other. java.lang.Integer 1.0 • static int compare(int x, int y) 7 returns a negative integer if x < y, zero if x and y are equal, and a positive integer otherwise. java.lang.Double 1.0 • static int compare(double x, double y) 1.4 returns a negative integer if x < y, zero if x and y are equal, and a positive integer otherwise. NOTE: According to the language standard: “The implementor must ensure sgn(x.compareTo(y)) = -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception if y.compareTo(x) throws an exception.)” Here, sgn is the sign of a number: sgn(n) is –1 if n is negative, 0 if n equals 0, and 1 if n is positive. In plain English, if you flip the parameters of compareTo, the sign (but not necessarily the actual value) of the result must also flip. As with the equals method, problems can arise when inheritance comes into play. Since Manager extends Employee, it implements Comparable<Employee> and not Comparable<Manager>. If Manager chooses to override compareTo, it must be prepared to compare managers to employees. It can’t simply cast an employee to a manager: From the Library of Hristo Dimov Hristov 6.1 Interfaces 303 class Manager extends Employee { public int compareTo(Employee other) { Manager otherManager = (Manager) other; // NO . . . } . . . } That violates the “antisymmetry” rule. If x is an Employee and y is a Manager, then the call x.compareTo(y) doesn’t throw an exception—it simply compares x and y as employees. But the reverse, y.compareTo(x), throws a ClassCastException. This is the same situation as with the equals method that we discussed in Chapter 5, and the remedy is the same. There are two distinct scenarios. If subclasses have different notions of comparison, then you should outlaw comparison of objects that belong to different classes. Each compareTo method should start out with the test if (getClass() != other.getClass()) throw new ClassCastException(); If there is a common algorithm for comparing subclass objects, simply provide a single compareTo method in the superclass and declare it as final. For example, suppose you want managers to be better than regular employees, regardless of salary. What about other subclasses such as Executive and Secretary? If you need to establish a pecking order, supply a method such as rank in the Employee class. Have each subclass override rank, and implement a single compareTo method that takes the rank values into account. 6.1.2 Properties of Interfaces Interfaces are not classes. In particular, you can never use the new operator to instantiate an interface: x = new Comparable(. . .); // ERROR However, even though you can’t construct interface objects, you can still declare interface variables. Comparable x; // OK An interface variable must refer to an object of a class that implements the interface: x = new Employee(. . .); // OK provided Employee implements Comparable From the Library of Hristo Dimov Hristov 304 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Next, just as you use instanceof to check whether an object is of a specific class, you can use instanceof to check whether an object implements an interface: if (anObject instanceof Comparable) { . . . } Just as you can build hierarchies of classes, you can extend interfaces. This allows for multiple chains of interfaces that go from a greater degree of generality to a greater degree of specialization. For example, suppose you had an interface called Moveable. public interface Moveable { void move(double x, double y); } Then, you could imagine an interface called Powered that extends it: public interface Powered extends Moveable { double milesPerGallon(); } Although you cannot put instance fields in an interface, you can supply constants in them. For example: public interface Powered extends Moveable { double milesPerGallon(); double SPEED_LIMIT = 95; // a public static final constant } Just as methods in an interface are automatically public, fields are always public static final. NOTE: It is legal to tag interface methods as public, and fields as public static final. Some programmers do that, either out of habit or for greater clarity. However, the Java Language Specification recommends that the redundant keywords not be supplied, and we follow that recommendation. Some interfaces define just constants and no methods. For example, the standard library contains an interface SwingConstants that defines constants NORTH, SOUTH, HORIZONTAL, and so on. Any class that chooses to implement the SwingConstants interface automatically inherits these constants. Its methods can simply refer to NORTH rather than the more cumbersome SwingConstants.NORTH. However, this use of interfaces seems rather degenerate, and we do not recommend it. From the Library of Hristo Dimov Hristov 6.1 Interfaces 305 While each class can have only one superclass, classes can implement multiple interfaces. This gives you the maximum amount of flexibility in defining a class’s behavior. For example, the Java programming language has an important interface built into it, called Cloneable. (We will discuss this interface in detail in Section 6.1.9, “Object Cloning,” on p. 314.) If your class implements Cloneable, the clone method in the Object class will make an exact copy of your class’s objects. If you want both cloneability and comparability, simply implement both interfaces. Use commas to separate the interfaces that you want to implement: class Employee implements Cloneable, Comparable 6.1.3 Interfaces and Abstract Classes If you read the section about abstract classes in Chapter 5, you may wonder why the designers of the Java programming language bothered with introducing the concept of interfaces. Why can’t Comparable simply be an abstract class: abstract class Comparable // why not? { public abstract int compareTo(Object other); } The Employee class compareTo method: would then simply extend this abstract class and supply the class Employee extends Comparable // why not? { public int compareTo(Object other) { . . . } } There is, unfortunately, a major problem with using an abstract base class to express a generic property. A class can only extend a single class. Suppose the Employee class already extends a different class, say, Person. Then it can’t extend a second class. class Employee extends Person, Comparable // ERROR But each class can implement as many interfaces as it likes: class Employee extends Person implements Comparable // OK Other programming languages, in particular C++, allow a class to have more than one superclass. This feature is called multiple inheritance. The designers of Java chose not to support multiple inheritance, because it makes the language either very complex (as in C++) or less efficient (as in Eiffel). From the Library of Hristo Dimov Hristov 306 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Instead, interfaces afford most of the benefits of multiple inheritance while avoiding the complexities and inefficiencies. C++ NOTE: C++ has multiple inheritance and all the complications that come with it, such as virtual base classes, dominance rules, and transverse pointer casts. Few C++ programmers use multiple inheritance, and some say it should never be used. Other programmers recommend using multiple inheritance only for the “mix-in” style of inheritance. In the mix-in style, a primary base class describes the parent object, and additional base classes (the so-called mix-ins) may supply auxiliary characteristics. That style is similar to a Java class with a single superclass and additional interfaces. 6.1.4 Static and Private Methods As of Java 8, you are allowed to add static methods to interfaces. There was never a technical reason why this should be outlawed. It simply seemed to be against the spirit of interfaces as abstract specifications. Up to now, it has been common to place static methods in companion classes. In the standard library, you’ll find pairs of interfaces and utility classes such as Collection/Collections or Path/Paths. You can construct a path to a file or directory from a URI, or from a sequence of strings, such as Paths.get("jdk-11", "conf", "security"). In Java 11, equivalent methods are provided in the Path interface: public interface Path { public static Path of(URI uri) { . . . } public static Path of(String first, String... more) { . . . } . . . } Then the Paths class is no longer necessary. Similarly, when you implement your own interfaces, there is no longer a reason to provide a separate companion class for utility methods. As of Java 9, methods in an interface can be private. A private method can be or an instance method. Since private methods can only be used in the methods of the interface itself, their use is limited to being helper methods for the other methods of the interface. static From the Library of Hristo Dimov Hristov 6.1 Interfaces 307 6.1.5 Default Methods You can supply a default implementation for any interface method. You must tag such a method with the default modifier. public interface Comparable<T> { default int compareTo(T other) { return 0; } // by default, all elements are the same } Of course, that is not very useful since every realistic implementation of Comparable would override this method. But there are other situations where default methods can be useful. For example, in Chapter 9 you will see an Iterator interface for visiting elements in a data structure. It declares a remove method as follows: public interface Iterator<E> { boolean hasNext(); E next(); default void remove() { throw new UnsupportedOperationException("remove"); } . . . } If you implement an iterator, you need to provide the hasNext and next methods. There are no defaults for these methods—they depend on the data structure that you are traversing. But if your iterator is read-only, you don’t have to worry about the remove method. A default method can call other methods. For example, a Collection interface can define a convenience method public interface Collection { int size(); // an abstract method default boolean isEmpty() { return size() == 0; } . . . } Then a programmer implementing Collection doesn’t have to worry about implementing an isEmpty method. NOTE: The Collection interface in the Java API does not actually do this. Instead, there is a class AbstractCollection that implements Collection and defines isEmpty in terms of size. Implementors of a collection are advised to extend AbstractCollection. That technique is obsolete. Just implement the methods in the interface. From the Library of Hristo Dimov Hristov 308 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes An important use for default methods is interface evolution. Consider, for example, the Collection interface that has been a part of Java for many years. Suppose that a long time ago, you provided a class public class Bag implements Collection Later, in Java 8, a stream method was added to the interface. Suppose the stream method was not a default method. Then the Bag class would no longer compile since it doesn’t implement the new method. Adding a nondefault method to an interface is not source-compatible. But suppose you don’t recompile the class and simply use an old JAR file containing it. The class will still load, even with the missing method. Programs can still construct Bag instances, and nothing bad will happen. (Adding a method to an interface is binary compatible.) However, if a program calls the stream method on a Bag instance, an AbstractMethodError occurs. Making the method a default method solves both problems. The Bag class will again compile. And if the class is loaded without being recompiled and the stream method is invoked on a Bag instance, the Collection.stream method is called. 6.1.6 Resolving Default Method Conflicts What happens if the exact same method is defined as a default method in one interface and then again as a method of a superclass or another interface? Languages such as Scala and C++ have complex rules for resolving such ambiguities. Fortunately, the rules in Java are much simpler. Here they are: 1. 2. Superclasses win. If a superclass provides a concrete method, default methods with the same name and parameter types are simply ignored. Interfaces clash. If an interface provides a default method, and another interface contains a method with the same name and parameter types (default or not), then you must resolve the conflict by overriding that method. Let’s look at the second rule. Consider two interfaces with a getName method: interface Person { default String getName() { return ""; }; } interface Named { default String getName() { return getClass().getName() + "_" + hashCode(); } } From the Library of Hristo Dimov Hristov 6.1 Interfaces 309 What happens if you form a class that implements both of them? class Student implements Person, Named { . . . } The class inherits two inconsistent getName methods provided by the Person and interfaces. Instead of choosing one over the other, the Java compiler reports an error and leaves it up to the programmer to resolve the ambiguity. Simply provide a getName method in the Student class. In that method, you can choose one of the two conflicting methods, like this: Named class Student implements Person, Named { public String getName() { return Person.super.getName(); } . . . } Now assume that the Named interface does not provide a default implementation for getName: interface Named { String getName(); } Can the Student class inherit the default method from the Person interface? This might be reasonable, but the Java designers decided in favor of uniformity. It doesn’t matter how two interfaces conflict. If at least one interface provides an implementation, the compiler reports an error, and the programmer must resolve the ambiguity. NOTE: Of course, if neither interface provides a default for a shared method, then we are in the situation before Java 8, and there is no conflict. An implementing class has two choices: implement the method, or leave it unimplemented. In the latter case, the class is itself abstract. We just discussed name clashes between two interfaces. Now consider a class that extends a superclass and implements an interface, inheriting the same method from both. For example, suppose that Person is a class and Student is defined as class Student extends Person implements Named { . . . } In that case, only the superclass method matters, and any default method from the interface is simply ignored. In our example, Student inherits the getName method from Person, and it doesn’t make any difference whether the Named interface provides a default for getName or not. This is the “class wins” rule. From the Library of Hristo Dimov Hristov 310 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes The “class wins” rule ensures compatibility with Java 7. If you add default methods to an interface, it has no effect on code that worked before there were default methods. CAUTION: You can never make a default method that redefines one of the methods in the Object class. For example, you can’t define a default method for toString or equals, even though that might be attractive for interfaces such as List. As a consequence of the “class wins” rule, such a method could never win against Object.toString or Objects.equals. 6.1.7 Interfaces and Callbacks A common pattern in programming is the callback pattern. In this pattern, you specify the action that should occur whenever a particular event happens. For example, you may want a particular action to occur when a button is clicked or a menu item is selected. However, as you have not yet seen how to implement user interfaces, we will consider a similar but simpler situation. The javax.swing package contains a Timer class that is useful if you want to be notified whenever a time interval has elapsed. For example, if a part of your program contains a clock, you can ask to be notified every second so that you can update the clock face. When you construct a timer, you set the time interval and tell it what it should do whenever the time interval has elapsed. How do you tell the timer what it should do? In many programming languages, you supply the name of a function that the timer should call periodically. However, the classes in the Java standard library take an object-oriented approach. You pass an object of some class. The timer then calls one of the methods on that object. Passing an object is more flexible than passing a function because the object can carry additional information. Of course, the timer needs to know what method to call. The timer requires that you specify an object of a class that implements the ActionListener interface of the java.awt.event package. Here is that interface: public interface ActionListener { void actionPerformed(ActionEvent event); } The timer calls the actionPerformed method when the time interval has expired. From the Library of Hristo Dimov Hristov 6.1 Interfaces 311 Suppose you want to print a message “At the tone, the time is . . .”, followed by a beep, once every second. You would define a class that implements the ActionListener interface. You would then place whatever statements you want to have executed inside the actionPerformed method. class TimePrinter implements ActionListener { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); Toolkit.getDefaultToolkit().beep(); } } Note the ActionEvent parameter of the actionPerformed method. This parameter gives information about the event, such as the time when the event happened. The call event.getWhen() returns the event time, measured in milliseconds since the “epoch” (January 1, 1970). By passing it to the static Instant.ofEpochMilli method, we get a more readable description. Next, construct an object of this class and pass it to the Timer constructor. var listener = new TimePrinter(); Timer t = new Timer(1000, listener); The first parameter of the Timer constructor is the time interval that must elapse between notifications, measured in milliseconds. We want to be notified every second. The second parameter is the listener object. Finally, start the timer. t.start(); Every second, a message like At the tone, the time is 2017-12-16T05:01:49.550Z is displayed, followed by a beep. Listing 6.3 puts the timer and its action listener to work. After the timer is started, the program puts up a message dialog and waits for the user to click the OK button to stop. While the program waits for the user, the current time is displayed every second. (If you omit the dialog, the program would terminate as soon as the main method exits.) From the Library of Hristo Dimov Hristov 312 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Listing 6.3 timer/TimerTest.java 1 package timer; 2 3 /** @version 1.02 2017-12-14 @author Cay Horstmann 4 5 6 */ 7 8 9 10 11 import import import import java.awt.*; java.awt.event.*; java.time.*; javax.swing.*; 12 13 14 15 16 17 public class TimerTest { public static void main(String[] args) { var listener = new TimePrinter(); 18 // construct a timer that calls the listener // once every second var timer = new Timer(1000, listener); timer.start(); 19 20 21 22 23 // keep program running until the user selects "OK" JOptionPane.showMessageDialog(null, "Quit program?"); System.exit(0); 24 25 26 } 27 28 } 29 30 31 32 33 34 35 36 37 38 class TimePrinter implements ActionListener { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); Toolkit.getDefaultToolkit().beep(); } } javax.swing.JOptionPane 1.2 • static void showMessageDialog(Component parent, Object message) displays a dialog box with a message prompt and an OK button. The dialog is centered over the parent component. If parent is null, the dialog is centered on the screen. From the Library of Hristo Dimov Hristov 6.1 Interfaces 313 javax.swing.Timer 1.2 • Timer(int interval, ActionListener listener) constructs a timer that notifies listener whenever interval milliseconds have elapsed. • void start() starts the timer. Once started, the timer calls actionPerformed on its listeners. • void stop() stops the timer. Once stopped, the timer no longer calls actionPerformed on its listeners. java.awt.Toolkit 1.0 • static Toolkit getDefaultToolkit() gets the default toolkit. A toolkit contains information about the GUI environment. • void beep() emits a beep sound. 6.1.8 The Comparator Interface In Section 6.1.1, “The Interface Concept,” on p. 296, you have seen how you can sort an array of objects, provided they are instances of classes that implement the Comparable interface. For example, you can sort an array of strings since the String class implements Comparable<String>, and the String.compareTo method compares strings in dictionary order. Now suppose we want to sort strings by increasing length, not in dictionary order. We can’t have the String class implement the compareTo method in two ways—and at any rate, the String class isn’t ours to modify. To deal with this situation, there is a second version of the Arrays.sort method whose parameters are an array and a comparator—an instance of a class that implements the Comparator interface. public interface Comparator<T> { int compare(T first, T second); } From the Library of Hristo Dimov Hristov 314 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes To compare strings by length, define a class that implements Comparator<String>: class LengthComparator implements Comparator<String> { public int compare(String first, String second) { return first.length() - second.length(); } } To actually do the comparison, you need to make an instance: var comp = new LengthComparator(); if (comp.compare(words[i], words[j]) > 0) . . . Contrast this call with words[i].compareTo(words[j]). The compare method is called on the comparator object, not the string itself. NOTE: Even though the LengthComparator object has no state, you still need to make an instance of it. You need the instance to call the compare method—it is not a static method. To sort an array, pass a LengthComparator object to the Arrays.sort method: String[] friends = { "Peter", "Paul", "Mary" }; Arrays.sort(friends, new LengthComparator()); Now the array is either ["Paul", "Mary", "Peter"] or ["Mary", "Paul", "Peter"]. You will see in Section 6.2, “Lambda Expressions,” on p. 322 how to use a Comparator much more easily with a lambda expression. 6.1.9 Object Cloning In this section, we discuss the Cloneable interface that indicates that a class has provided a safe clone method. Since cloning is not all that common, and the details are quite technical, you may just want to glance at this material until you need it. To understand what cloning means, recall what happens when you make a copy of a variable holding an object reference. The original and the copy are references to the same object (see Figure 6.1). This means a change to either variable also affects the other. var original = new Employee("John Public", 50000); Employee copy = original; copy.raiseSalary(10); // oops--also changed original From the Library of Hristo Dimov Hristov 6.1 Interfaces 315 Figure 6.1 Copying and cloning If you would like copy to be a new object that begins its life being identical to original but whose state can diverge over time, use the clone method. Employee copy = original.clone(); copy.raiseSalary(10); // OK--original unchanged But it isn’t quite so simple. The clone method is a protected method of Object, which means that your code cannot simply call it. Only the Employee class can clone Employee objects. There is a reason for this restriction. Think about the way in which the Object class can implement clone. It knows nothing about the object at all, so it can make only a field-by-field copy. If all data fields From the Library of Hristo Dimov Hristov 316 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes in the object are numbers or other basic types, copying the fields is just fine. But if the object contains references to subobjects, then copying the field gives you another reference to the same subobject, so the original and the cloned objects still share some information. To visualize that, consider the Employee class that was introduced in Chapter 4. Figure 6.2 shows what happens when you use the clone method of the Object class to clone such an Employee object. As you can see, the default cloning operation is “shallow”—it doesn’t clone objects that are referenced inside other objects. (The figure shows a shared Date object. For reasons that will become clear shortly, this example uses a version of the Employee class in which the hire day is represented as a Date.) Figure 6.2 A shallow copy Does it matter if the copy is shallow? It depends. If the subobject shared between the original and the shallow clone is immutable, then the sharing is safe. This certainly happens if the subobject belongs to an immutable class, such as String. Alternatively, the subobject may simply remain constant throughout the lifetime of the object, with no mutators touching it and no methods yielding a reference to it. Quite frequently, however, subobjects are mutable, and you must redefine the clone method to make a deep copy that clones the subobjects as well. In From the Library of Hristo Dimov Hristov 6.1 Interfaces 317 our example, the hireDay field is a Date, which is mutable, so it too must be cloned. (For that reason, this example uses a field of type Date, not LocalDate, to demonstrate the cloning process. Had hireDay been an instance of the immutable LocalDate class, no further action would have been required.) For every class, you need to decide whether 1. 2. 3. The default clone method is good enough; The default clone method can be patched up by calling clone on the mutable subobjects; or clone should not be attempted. The third option is actually the default. To choose either the first or the second option, a class must 1. 2. Implement the Cloneable interface; and Redefine the clone method with the public access modifier. NOTE: The clone method is declared protected in the Object class, so that your code can’t simply call anObject.clone(). But aren’t protected methods accessible from any subclass, and isn’t every class a subclass of Object? Fortunately, the rules for protected access are more subtle (see Chapter 5). A subclass can call a protected clone method only to clone its own objects. You must redefine clone to be public to allow objects to be cloned by any method. In this case, the appearance of the Cloneable interface has nothing to do with the normal use of interfaces. In particular, it does not specify the clone method—that method is inherited from the Object class. The interface merely serves as a tag, indicating that the class designer understands the cloning process. Objects are so paranoid about cloning that they generate a checked exception if an object requests cloning but does not implement that interface. NOTE: The Cloneable interface is one of a handful of tagging interfaces that Java provides. (Some programmers call them marker interfaces.) Recall that the usual purpose of an interface such as Comparable is to ensure that a class implements a particular method or set of methods. A tagging interface has no methods; its only purpose is to allow the use of instanceof in a type inquiry: if (obj instanceof Cloneable) . . . We recommend that you do not use tagging interfaces in your own programs. From the Library of Hristo Dimov Hristov 318 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Even if the default (shallow copy) implementation of clone is adequate, you still need to implement the Cloneable interface, redefine clone to be public, and call super.clone(). Here is an example: class Employee implements Cloneable { // public access, change return type public Employee clone() throws CloneNotSupportedException { return (Employee) super.clone(); } . . . } NOTE: Up to Java 1.4, the clone method always had return type Object. Nowadays, you can specify the correct return type for your clone methods. This is an example of covariant return types (see Chapter 5). The clone method that you just saw adds no functionality to the shallow copy provided by Object.clone. It merely makes the method public. To make a deep copy, you have to work harder and clone the mutable instance fields. Here is an example of a clone method that creates a deep copy: class Employee implements Cloneable { . . . public Employee clone() throws CloneNotSupportedException { // call Object.clone() Employee cloned = (Employee) super.clone(); // clone mutable fields cloned.hireDay = (Date) hireDay.clone(); return cloned; } } The clone method of the Object class threatens to throw a CloneNotSupportedException— it does that whenever clone is invoked on an object whose class does not implement the Cloneable interface. Of course, the Employee and Date classes implement the Cloneable interface, so the exception won’t be thrown. However, the compiler does not know that. Therefore, we declared the exception: public Employee clone() throws CloneNotSupportedException From the Library of Hristo Dimov Hristov 6.1 Interfaces 319 NOTE: Would it be better to catch the exception instead? (See Chapter 7 for details on catching exceptions.) public Employee clone() { try { Employee cloned = (Employee) super.clone(); . . . } catch (CloneNotSupportedException e) { return null; } // this won't happen, since we are Cloneable } This is appropriate for final classes. Otherwise, it is better to leave the throws specifier in place. That gives subclasses the option of throwing a CloneNotSupportedException if they can’t support cloning. You have to be careful about cloning of subclasses. For example, once you have defined the clone method for the Employee class, anyone can use it to clone Manager objects. Can the Employee clone method do the job? It depends on the fields of the Manager class. In our case, there is no problem because the bonus field has primitive type. But Manager might have acquired fields that require a deep copy or are not cloneable. There is no guarantee that the implementor of the subclass has fixed clone to do the right thing. For that reason, the clone method is declared as protected in the Object class. But you don’t have that luxury if you want the users of your classes to invoke clone. Should you implement clone in your own classes? If your clients need to make deep copies, then you probably should. Some authors feel that you should avoid clone altogether and instead implement another method for the same purpose. We agree that clone is rather awkward, but you’ll run into the same issues if you shift the responsibility to another method. At any rate, cloning is less common than you may think. Less than 5 percent of the classes in the standard library implement clone. The program in Listing 6.4 clones an instance of the class Employee (Listing 6.5), then invokes two mutators. The raiseSalary method changes the value of the salary field, whereas the setHireDay method changes the state of the hireDay field. Neither mutation affects the original object because clone has been defined to make a deep copy. From the Library of Hristo Dimov Hristov 320 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes NOTE: All array types have a clone method that is public, not protected. You can use it to make a new array that contains copies of all elements. For example: int[] luckyNumbers = { 2, 3, 5, 7, 11, 13 }; int[] cloned = luckyNumbers.clone(); cloned[5] = 12; // doesn't change luckyNumbers[5] NOTE: Chapter 2 of Volume II shows an alternate mechanism for cloning objects, using the object serialization feature of Java. That mechanism is easy to implement and safe, but not very efficient. Listing 6.4 clone/CloneTest.java 1 package clone; 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 /** * This program demonstrates cloning. * @version 1.11 2018-03-16 * @author Cay Horstmann */ public class CloneTest { public static void main(String[] args) throws CloneNotSupportedException { var original = new Employee("John Q. Public", 50000); original.setHireDay(2000, 1, 1); Employee copy = original.clone(); copy.raiseSalary(10); copy.setHireDay(2002, 12, 31); System.out.println("original=" + original); System.out.println("copy=" + copy); } } Listing 6.5 clone/Employee.java 1 package clone; 2 3 4 import java.util.Date; import java.util.GregorianCalendar; 5 6 7 public class Employee implements Cloneable { From the Library of Hristo Dimov Hristov 6.1 Interfaces 321 private String name; private double salary; private Date hireDay; 8 9 10 11 public Employee(String name, double salary) { this.name = name; this.salary = salary; hireDay = new Date(); } 12 13 14 15 16 17 18 public Employee clone() throws CloneNotSupportedException { // call Object.clone() Employee cloned = (Employee) super.clone(); 19 20 21 22 23 // clone mutable fields cloned.hireDay = (Date) hireDay.clone(); 24 25 26 return cloned; 27 } 28 29 /** * Set the hire day to a given date. * @param year the year of the hire day * @param month the month of the hire day * @param day the day of the hire day */ public void setHireDay(int year, int month, int day) { Date newHireDay = new GregorianCalendar(year, month - 1, day).getTime(); 30 31 32 33 34 35 36 37 38 39 // example of instance field mutation hireDay.setTime(newHireDay.getTime()); 40 41 } 42 43 public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } 44 45 46 47 48 49 public String toString() { return "Employee[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay + "]"; } 50 51 52 53 54 } From the Library of Hristo Dimov Hristov 322 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes 6.2 Lambda Expressions In the following sections, you will learn how to use lambda expressions for defining blocks of code with a concise syntax, and how to write code that consumes lambda expressions. 6.2.1 Why Lambdas? A lambda expression is a block of code that you can pass around so it can be executed later, once or multiple times. Before getting into the syntax (or even the curious name), let’s step back and observe where we have used such code blocks in Java. In Section 6.1.7, “Interfaces and Callbacks,” on p. 310, you saw how to do work in timed intervals. Put the work into the actionPerformed method of an ActionListener: class Worker implements ActionListener { public void actionPerformed(ActionEvent event) { // do some work } } Then, when you want to repeatedly execute this code, you construct an instance of the Worker class. You then submit the instance to a Timer object. The key point is that the actionPerformed method contains code that you want to execute later. Or consider sorting with a custom comparator. If you want to sort strings by length instead of the default dictionary order, you can pass a Comparator object to the sort method: class LengthComparator implements Comparator<String> { public int compare(String first, String second) { return first.length() - second.length(); } } . . . Arrays.sort(strings, new LengthComparator()); The compare method isn’t called right away. Instead, the sort method keeps calling the compare method, rearranging the elements if they are out of order, until the array is sorted. You give the sort method a snippet of code needed From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 323 to compare elements, and that code is integrated into the rest of the sorting logic, which you’d probably not care to reimplement. Both examples have something in common. A block of code was passed to someone—a timer, or a sort method. That code block was called at some later time. Up to now, giving someone a block of code hasn’t been easy in Java. You couldn’t just pass code blocks around. Java is an object-oriented language, so you had to construct an object belonging to a class that has a method with the desired code. In other languages, it is possible to work with blocks of code directly. The Java designers have resisted adding this feature for a long time. After all, a great strength of Java is its simplicity and consistency. A language can become an unmaintainable mess if it includes every feature that yields marginally more concise code. However, in those other languages it isn’t just easier to spawn a thread or to register a button click handler; large swaths of their APIs are simpler, more consistent, and more powerful. In Java, one could have written similar APIs taking objects of classes that implement a particular interface, but such APIs would be unpleasant to use. For some time, the question was not whether to augment Java for functional programming, but how to do it. It took several years of experimentation before a design emerged that is a good fit for Java. In the next section, you will see how you can work with blocks of code in Java. 6.2.2 The Syntax of Lambda Expressions Consider again the sorting example from the preceding section. We pass code that checks whether one string is shorter than another. We compute first.length() - second.length() What are first and second? They are both strings. Java is a strongly typed language, and we must specify that as well: (String first, String second) -> first.length() - second.length() You have just seen your first lambda expression. Such an expression is simply a block of code, together with the specification of any variables that must be passed to the code. Why the name? Many years ago, before there were any computers, the logician Alonzo Church wanted to formalize what it means for a mathematical function to be effectively computable. (Curiously, there are functions that are known to exist, but nobody knows how to compute their values.) He used the Greek From the Library of Hristo Dimov Hristov 324 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes letter lambda (λ) to mark parameters. Had he known about the Java API, he would have written λfirst.λsecond.first.length() - second.length() NOTE: Why the letter λ? Did Church run out of other letters of the alphabet? Actually, the venerable Principia Mathematica used the ^ accent to denote free variables, which inspired Church to use an uppercase lambda Λ for parameters. But in the end, he switched to the lowercase version. Ever since, an expression with parameter variables has been called a lambda expression. You have just seen one form of lambda expressions in Java: parameters, the -> arrow, and an expression. If the code carries out a computation that doesn’t fit in a single expression, write it exactly like you would have written a method: enclosed in {} and with explicit return statements. For example, (String first, String second) -> { if (first.length() < second.length()) return -1; else if (first.length() > second.length()) return 1; else return 0; } If a lambda expression has no parameters, you still supply empty parentheses, just as with a parameterless method: () -> { for (int i = 100; i >= 0; i--) System.out.println(i); } If the parameter types of a lambda expression can be inferred, you can omit them. For example, Comparator<String> comp = (first, second) // same as (String first, String second) -> first.length() - second.length(); Here, the compiler can deduce that first and second must be strings because the lambda expression is assigned to a string comparator. (We will have a closer look at this assignment in the next section.) If a method has a single parameter with inferred type, you can even omit the parentheses: ActionListener listener = event -> System.out.println("The time is " + Instant.ofEpochMilli(event.getWhen())); // instead of (event) -> . . . or (ActionEvent event) -> . . . From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 325 You never specify the result type of a lambda expression. It is always inferred from context. For example, the expression (String first, String second) -> first.length() - second.length() can be used in a context where a result of type int is expected. NOTE: It is illegal for a lambda expression to return a value in some branches but not in others. For example, (int x) -> { if (x >= 0) return 1; } is invalid. The program in Listing 6.6 shows how to use lambda expressions for a comparator and an action listener. Listing 6.6 lambda/LambdaTest.java 1 package lambda; 2 3 import java.util.*; 4 5 6 import javax.swing.*; import javax.swing.Timer; 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 /** * This program demonstrates the use of lambda expressions. * @version 1.0 2015-05-12 * @author Cay Horstmann */ public class LambdaTest { public static void main(String[] args) { var planets = new String[] { "Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune" }; System.out.println(Arrays.toString(planets)); System.out.println("Sorted in dictionary order:"); Arrays.sort(planets); System.out.println(Arrays.toString(planets)); System.out.println("Sorted by length:"); Arrays.sort(planets, (first, second) -> first.length() - second.length()); System.out.println(Arrays.toString(planets)); 26 27 28 29 var timer = new Timer(1000, event -> System.out.println("The time is " + new Date())); timer.start(); 30 (Continues) From the Library of Hristo Dimov Hristov 326 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Listing 6.6 (Continued) // keep program running until user selects "OK" JOptionPane.showMessageDialog(null, "Quit program?"); System.exit(0); 31 32 33 } 34 35 } 6.2.3 Functional Interfaces As we discussed, there are many existing interfaces in Java that encapsulate blocks of code, such as ActionListener or Comparator. Lambdas are compatible with these interfaces. You can supply a lambda expression whenever an object of an interface with a single abstract method is expected. Such an interface is called a functional interface. NOTE: You may wonder why a functional interface must have a single abstract method. Aren’t all methods in an interface abstract? Actually, it has always been possible for an interface to redeclare methods from the Object class such as toString or clone, and these declarations do not make the methods abstract. (Some interfaces in the Java API redeclare Object methods in order to attach javadoc comments. Check out the Comparator API for an example.) More importantly, as you saw in Section 6.1.5, “Default Methods,” on p. 307, interfaces can declare nonabstract methods. To demonstrate the conversion to a functional interface, consider the Arrays.sort method. Its second parameter requires an instance of Comparator, an interface with a single method. Simply supply a lambda: Arrays.sort(words, (first, second) -> first.length() - second.length()); Behind the scenes, the Arrays.sort method receives an object of some class that implements Comparator<String>. Invoking the compare method on that object executes the body of the lambda expression. The management of these objects and classes is completely implementation-dependent, and it can be much more efficient than using traditional inner classes. It is best to think of a lambda expression as a function, not an object, and to accept that it can be passed to a functional interface. This conversion to interfaces is what makes lambda expressions so compelling. The syntax is short and simple. Here is another example: From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 327 var timer = new Timer(1000, event -> { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); Toolkit.getDefaultToolkit().beep(); }); That’s a lot easier to read than the alternative with a class that implements the ActionListener interface. In fact, conversion to a functional interface is the only thing that you can do with a lambda expression in Java. In other programming languages that support function literals, you can declare function types such as (String, String) -> int, declare variables of those types, and use the variables to save function expressions. However, the Java designers decided to stick with the familiar concept of interfaces instead of adding function types to the language. NOTE: You can’t even assign a lambda expression to a variable of type Object—Object is not a functional interface. The Java API defines a number of very generic functional interfaces in the java.util.function package. One of the interfaces, BiFunction<T, U, R>, describes functions with parameter types T and U and return type R. You can save our string comparison lambda in a variable of that type: BiFunction<String, String, Integer> comp = (first, second) -> first.length() - second.length(); However, that does not help you with sorting. There is no Arrays.sort method that wants a BiFunction. If you have used a functional programming language before, you may find this curious. But for Java programmers, it’s pretty natural. An interface such as Comparator has a specific purpose, not just a method with given parameter and return types. When you want to do something with lambda expressions, you still want to keep the purpose of the expression in mind, and have a specific functional interface for it. A particularly useful interface in the java.util.function package is Predicate: public interface Predicate<T> { boolean test(T t); // additional default and static methods } From the Library of Hristo Dimov Hristov 328 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes The ArrayList class has a removeIf method whose parameter is a Predicate. It is specifically designed to pass a lambda expression. For example, the following statement removes all null values from an array list: list.removeIf(e -> e == null); Another useful functional interface is Supplier<T>: public interface Supplier<T> { T get(); } A supplier has no arguments and yields a value of type T when it is called. Suppliers are used for lazy evaluation. For example, consider the call LocalDate hireDay = Objects.requireNonNullElse(day, LocalDate.of(1970, 1, 1)); This is not optimal. We expect that day is rarely null, so we only want to construct the default LocalDate when necessary. By using the supplier, we can defer the computation: LocalDate hireDay = Objects.requireNonNullElseGet(day, () -> LocalDate.of(1970, 1, 1)); The requireNonNullElseGet method only calls the supplier when the value is needed. 6.2.4 Method References Sometimes, a lambda expression involves a single method. For example, suppose you simply want to print the event object whenever a timer event occurs. Of course, you could call var timer = new Timer(1000, event -> System.out.println(event)); It would be nicer if you could just pass the println method to the Timer constructor. Here is how you do that: var timer = new Timer(1000, System.out::println); The expression System.out::println is a method reference. It directs the compiler to produce an instance of a functional interface, overriding the single abstract method of the interface to call the given method. In this example, an ActionListener is produced whose actionPerformed(ActionEvent e) method calls System.out.println(e). From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 329 NOTE: Like a lambda expression, a method reference is not an object. It gives rise to an object when assigned to a variable whose type is a functional interface. NOTE: There are ten overloaded println methods in the PrintStream class (of which System.out is an instance). The compiler needs to figure out which one to use, depending on context. In our example, the method reference System.out::println must be turned into an ActionListener instance with a method void actionPerformed(ActionEvent e) The println(Object x) method is selected from the ten overloaded println methods since Object is the best match for ActionEvent. When the actionPerformed method is called, the event object is printed. Now suppose we assign the same method reference to a different functional interface: Runnable task = System.out::println; The Runnable functional interface has a single abstract method with no parameters void run() In this case, the println() method with no parameters is chosen. Calling task.run() prints a blank line to System.out. As another example, suppose you want to sort strings regardless of letter case. You can pass this method expression: Arrays.sort(strings, String::compareToIgnoreCase) As you can see from these examples, the :: operator separates the method name from the name of an object or class. There are three variants: 1. object::instanceMethod 2. 3. Class::instanceMethod Class::staticMethod In the first variant, the method reference is equivalent to a lambda expression whose parameters are passed to the method. In the case of System.out::println, the object is System.out, and the method expression is equivalent to x -> System.out.println(x). In the second variant, the first parameter becomes the implicit parameter of the method. For example, String::compareToIgnoreCase is the same as (x, y) -> x.compareToIgnoreCase(y). From the Library of Hristo Dimov Hristov 330 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes In the third variant, all parameters are passed to the static method: Math::pow is equivalent to (x, y) -> Math.pow(x, y). Table 6.1 walks you through additional examples. Note that a lambda expression can only be rewritten as a method reference if the body of the lambda expression calls a single method and doesn’t do anything else. Consider the lambda expression s -> s.length() == 0 There is a single method call. But there is also a comparison, so you can’t use a method reference here. NOTE: When there are multiple overloaded methods with the same name, the compiler will try to find from the context which one you mean. For example, there are two versions of the Math.max method, one for integers and one for double values. Which one gets picked depends on the method parameters of the functional interface to which Math::max is converted. Just like lambda expressions, method references don’t live in isolation. They are always turned into instances of functional interfaces. NOTE: Sometimes, the API contains methods that are specifically intended to be used as method references. For example, the Objects class has a method isNull to test whether an object reference is null. At first glance, this doesn’t seem useful because the test obj == null is easier to read than Objects.isNull(obj). But you can pass the method reference to any method with a Predicate parameter. For example, to remove all null references from a list, you can call list.removeIf(Objects::isNull); // A bit easier to read than list.removeIf(e -> e == null); NOTE: There is a tiny difference between a method reference with an object and its equivalent lambda expression. Consider a method reference such as separator::equals. If separator is null, forming separator::equals immediately throws a NullPointerException. The lambda expression x -> separator.equals(x) only throws a NullPointerException if it is invoked. You can capture the this parameter in a method reference. For example, this::equals is the same as x -> this.equals(x). It is also valid to use super. The method expression super::instanceMethod From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 331 Table 6.1 Method Reference Examples Method Reference Equivalent Lambda Expression Notes separator::equals x -> separator.equals(x) This is a method expression with an object and an instance method. The lambda parameter is passed as the explicit parameter of the method. String::trim x -> x.trim() This is a method expression with a class and an instance method. The lambda parameter becomes the implicit parameter. String::concat (x, y) -> x.concat(y) Again, we have an instance method, but this time, with an explicit parameter. As before, the first lambda parameter becomes the implicit parameter, and the remaining ones are passed to the method. Integer.valueOf x -> Integer.valueOf(x) This is a method expression with a static method. The lambda parameter is passed to the static method. Integer.sum (x, y) -> Integer.sum(x, y) This is another static method, but this time with two parameters. Both lambda parameters are passed to the static method. The Integer.sum method was specifically created to be used as a method reference. As a lambda, you could just write (x, y) -> x + y. Integer::new x -> new Integer(x) This is a constructor reference—see Section 6.2.5. The lambda parameters are passed to the constructor. Integer[]::new n -> new Integer[n] This is an array constructor reference—see Section 6.2.5. The lambda parameter is the array length. From the Library of Hristo Dimov Hristov 332 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes uses this as the target and invokes the superclass version of the given method. Here is an artificial example that shows the mechanics: class Greeter { public void greet(ActionEvent event) { System.out.println("Hello, the time is " + Instant.ofEpochMilli(event.getWhen())); } } class RepeatedGreeter extends Greeter { public void greet(ActionEvent event) { var timer = new Timer(1000, super::greet); timer.start(); } } When the RepeatedGreeter.greet method starts, a Timer is constructed that executes the super::greet method on every timer tick. 6.2.5 Constructor References Constructor references are just like method references, except that the name of the method is new. For example, Person::new is a reference to a Person constructor. Which constructor? It depends on the context. Suppose you have a list of strings. Then you can turn it into an array of Person objects, by calling the constructor on each of the strings, with the following invocation: ArrayList<String> names = . . .; Stream<Person> stream = names.stream().map(Person::new); List<Person> people = stream.collect(Collectors.toList()); We will discuss the details of the stream, map, and collect methods in Chapter 1 of Volume II. For now, what’s important is that the map method calls the Person(String) constructor for each list element. If there are multiple Person constructors, the compiler picks the one with a String parameter because it infers from the context that the constructor is called with a string. You can form constructor references with array types. For example, int[]::new is a constructor reference with one parameter: the length of the array. It is equivalent to the lambda expression x -> new int[x]. Array constructor references are useful to overcome a limitation of Java. It is not possible to construct an array of a generic type T. The expression new T[n] is an error since it would be erased to new Object[n]. That is a problem for library From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 333 authors. For example, suppose we want to have an array of Person objects. The Stream interface has a toArray method that returns an Object array: Object[] people = stream.toArray(); But that is unsatisfactory. The user wants an array of references to Person, not references to Object. The stream library solves that problem with constructor references. Pass Person[]::new to the toArray method: Person[] people = stream.toArray(Person[]::new); The toArray method invokes this constructor to obtain an array of the correct type. Then it fills and returns the array. 6.2.6 Variable Scope Often, you want to be able to access variables from an enclosing method or class in a lambda expression. Consider this example: public static void repeatMessage(String text, int delay) { ActionListener listener = event -> { System.out.println(text); Toolkit.getDefaultToolkit().beep(); }; new Timer(delay, listener).start(); } Consider a call repeatMessage("Hello", 1000); // prints Hello every 1,000 milliseconds Now look at the variable text inside the lambda expression. Note that this variable is not defined in the lambda expression. Instead, it is a parameter variable of the repeatMessage method. If you think about it, something nonobvious is going on here. The code of the lambda expression may run long after the call to repeatMessage has returned and the parameter variables are gone. How does the text variable stay around? To understand what is happening, we need to refine our understanding of a lambda expression. A lambda expression has three ingredients: 1. 2. 3. A block of code Parameters Values for the free variables—that is, the variables that are not parameters and not defined inside the code From the Library of Hristo Dimov Hristov 334 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes In our example, the lambda expression has one free variable, text. The data structure representing the lambda expression must store the values for the free variables—in our case, the string "Hello". We say that such values have been captured by the lambda expression. (It’s an implementation detail how that is done. For example, one can translate a lambda expression into an object with a single method, so that the values of the free variables are copied into instance variables of that object.) NOTE: The technical term for a block of code together with the values of the free variables is a closure. If someone gloats that their language has closures, rest assured that Java has them as well. In Java, lambda expressions are closures. As you have seen, a lambda expression can capture the value of a variable in the enclosing scope. In Java, to ensure that the captured value is welldefined, there is an important restriction. In a lambda expression, you can only reference variables whose value doesn’t change. For example, the following is illegal: public static void countDown(int start, int delay) { ActionListener listener = event -> { start--; // ERROR: Can't mutate captured variable System.out.println(start); }; new Timer(delay, listener).start(); } There is a reason for this restriction. Mutating variables in a lambda expression is not safe when multiple actions are executed concurrently. This won’t happen for the kinds of actions that we have seen so far, but in general, it is a serious problem. See Chapter 12 for more information on this important issue. It is also illegal to refer, in a lambda expression, to a variable that is mutated outside. For example, the following is illegal: public static void repeat(String text, int count) { for (int i = 1; i <= count; i++) { ActionListener listener = event -> { System.out.println(i + ": " + text); // ERROR: Cannot refer to changing i }; From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 335 new Timer(1000, listener).start(); } } The rule is that any captured variable in a lambda expression must be effectively final. An effectively final variable is a variable that is never assigned a new value after it has been initialized. In our case, text always refers to the same String object, and it is OK to capture it. However, the value of i is mutated, and therefore i cannot be captured. The body of a lambda expression has the same scope as a nested block. The same rules for name conflicts and shadowing apply. It is illegal to declare a parameter or a local variable in the lambda that has the same name as a local variable. Path first = Path.of("/usr/bin"); Comparator<String> comp = (first, second) -> first.length() - second.length(); // ERROR: Variable first already defined Inside a method, you can’t have two local variables with the same name, and therefore, you can’t introduce such variables in a lambda expression either. When you use the this keyword in a lambda expression, you refer to the this parameter of the method that creates the lambda. For example, consider public class Application { public void init() { ActionListener listener = event -> { System.out.println(this.toString()); . . . } . . . } } The expression this.toString() calls the toString method of the Application object, not the ActionListener instance. There is nothing special about the use of this in a lambda expression. The scope of the lambda expression is nested inside the init method, and this has the same meaning anywhere in that method. 6.2.7 Processing Lambda Expressions Up to now, you have seen how to produce lambda expressions and pass them to a method that expects a functional interface. Now let us see how to write methods that can consume lambda expressions. From the Library of Hristo Dimov Hristov 336 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes The point of using lambdas is deferred execution. After all, if you wanted to execute some code right now, you’d do that, without wrapping it inside a lambda. There are many reasons for executing code later, such as: • Running the code in a separate thread • Running the code multiple times • Running the code at the right point in an algorithm (for example, the comparison operation in sorting) • Running the code when something happens (a button was clicked, data has arrived, and so on) • Running the code only when necessary Let’s look at a simple example. Suppose you want to repeat an action n times. The action and the count are passed to a repeat method: repeat(10, () -> System.out.println("Hello, World!")); To accept the lambda, we need to pick (or, in rare cases, provide) a functional interface. Table 6.2 lists the most important functional interfaces that are provided in the Java API. In this case, we can use the Runnable interface: public static void repeat(int n, Runnable action) { for (int i = 0; i < n; i++) action.run(); } Note that the body of the lambda expression is executed when action.run() is called. Now let’s make this example a bit more sophisticated. We want to tell the action in which iteration it occurs. For that, we need to pick a functional interface that has a method with an int parameter and a void return. The standard interface for processing int values is public interface IntConsumer { void accept(int value); } Here is the improved version of the repeat method: public static void repeat(int n, IntConsumer action) { for (int i = 0; i < n; i++) action.accept(i); } From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 337 And here is how you call it: repeat(10, i -> System.out.println("Countdown: " + (9 - i))); Table 6.2 Common Functional Interfaces Functional Interface Parameter Types Return Type Abstract Method Name Description Other Methods Runnable none void run Runs an action without arguments or return value Supplier<T> none T get Supplies a value of type T Consumer<T> T void accept Consumes a value of type T andThen BiConsumer<T, U> T, U void accept Consumes values of types T and U andThen Function<T, R> T R apply A function with argument of type T compose, andThen, identity BiFunction<T, U, R> T, U R apply A function with arguments of types T and U andThen UnaryOperator<T> T T apply A unary operator on the type T compose, andThen, identity BinaryOperator<T> T, T T apply A binary operator on the type T andThen, maxBy, minBy Predicate<T> T boolean test A boolean-valued function and, or, negate, isEqual BiPredicate<T, U> T, U boolean test A boolean-valued function with two arguments and, or, negate From the Library of Hristo Dimov Hristov 338 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Table 6.3 lists the 34 available specializations for primitive types int, long, and As you will see in Chapter 8, it is more efficient to use these specializations than the generic interfaces. For that reason, I used an IntConsumer instead of a Consumer<Integer> in the example of the preceding section. double. Table 6.3 Functional Interfaces for Primitive Types p, q is int, long, double; P, Q is Int, Long, Double Functional Interface Parameter Types Return Type Abstract Method Name BooleanSupplier none boolean getAsBoolean PSupplier none p getAsP PConsumer p void accept ObjPConsumer<T> T, p void accept PFunction<T> p T apply PToQFunction p q applyAsQ ToPFunction<T> T p applyAsP ToPBiFunction<T, U> T, U p applyAsP PUnaryOperator p p applyAsP PBinaryOperator p, p p applyAsP PPredicate p boolean test TIP: It is a good idea to use an interface from Tables 6.2 or 6.3 whenever you can. For example, suppose you write a method to process files that match a certain criterion. There is a legacy interface java.io.FileFilter, but it is better to use the standard Predicate<File>. The only reason not to do so would be if you already have many useful methods producing FileFilter instances. NOTE: Most of the standard functional interfaces have nonabstract methods for producing or combining functions. For example, Predicate.isEqual(a) is the same as a::equals, but it also works if a is null. There are default methods and, or, negate for combining predicates. For example, Predicate.isEqual(a).or( Predicate.isEqual(b)) is the same as x -> a.equals(x) || b.equals(x). From the Library of Hristo Dimov Hristov 6.2 Lambda Expressions 339 NOTE: If you design your own interface with a single abstract method, you can tag it with the @FunctionalInterface annotation. This has two advantages. The compiler gives an error message if you accidentally add another abstract method. And the javadoc page includes a statement that your interface is a functional interface. It is not required to use the annotation. Any interface with a single abstract method is, by definition, a functional interface. But using the @FunctionalInterface annotation is a good idea. 6.2.8 More about Comparators The Comparator interface has a number of convenient static methods for creating comparators. These methods are intended to be used with lambda expressions or method references. The static comparing method takes a “key extractor” function that maps a type to a comparable type (such as String). The function is applied to the objects to be compared, and the comparison is then made on the returned keys. For example, suppose you have an array of Person objects. Here is how you can sort them by name: T Arrays.sort(people, Comparator.comparing(Person::getName)); This is certainly much easier than implementing a Comparator by hand. Moreover, the code is clearer since it is obvious that we want to compare people by name. You can chain comparators with the thenComparing method for breaking ties. For example, Arrays.sort(people, Comparator.comparing(Person::getLastName) .thenComparing(Person::getFirstName)); If two people have the same last name, then the second comparator is used. There are a few variations of these methods. You can specify a comparator to be used for the keys that the comparing and thenComparing methods extract. For example, here we sort people by the length of their names: Arrays.sort(people, Comparator.comparing(Person::getName, (s, t) -> Integer.compare(s.length(), t.length()))); From the Library of Hristo Dimov Hristov 340 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Moreover, both the comparing and thenComparing methods have variants that avoid boxing of int, long, or double values. An easier way of producing the preceding operation would be Arrays.sort(people, Comparator.comparingInt(p -> p.getName().length())); If your key function can return null, you will like the nullsFirst and nullsLast adapters. These static methods take an existing comparator and modify it so that it doesn’t throw an exception when encountering null values but ranks them as smaller or larger than regular values. For example, suppose getMiddleName returns a null when a person has no middle name. Then you can use Comparator.comparing(Person::getMiddleName, Comparator.nullsFirst(. . .)). The nullsFirst method needs a comparator—in this case, one that compares two strings. The naturalOrder method makes a comparator for any class implementing Comparable. A Comparator.<String>naturalOrder() is what we need. Here is the complete call for sorting by potentially null middle names. I use a static import of java.util.Comparator.*, to make the expression more legible. Note that the type for naturalOrder is inferred. Arrays.sort(people, comparing(Person::getMiddleName, nullsFirst(naturalOrder()))); The static reverseOrder method gives the reverse of the natural order. To reverse any comparator, use the reversed instance method. For example, naturalOrder().reversed() is the same as reverseOrder(). 6.3 Inner Classes An inner class is a class that is defined inside another class. Why would you want to do that? There are two reasons: • Inner classes can be hidden from other classes in the same package. • Inner class methods can access the data from the scope in which they are defined—including the data that would otherwise be private. Inner classes used to be very important for concisely implementing callbacks, but nowadays lambda expressions do a much better job. Still, inner classes can be very useful for structuring your code. The following sections walk you through all the details. From the Library of Hristo Dimov Hristov 6.3 Inner Classes 341 C++ NOTE: C++ has nested classes. A nested class is contained inside the scope of the enclosing class. Here is a typical example: A linked list class defines a class to hold the links, and a class to define an iterator position. class LinkedList { public: class Iterator // a nested class { public: void insert(int x); int erase(); . . . private: Link* current; LinkedList* owner; }; . . . private: Link* head; Link* tail; }; Nested classes are similar to inner classes in Java. However, the Java inner classes have an additional feature that makes them richer and more useful than nested classes in C++. An object that comes from an inner class has an implicit reference to the outer class object that instantiated it. Through this pointer, it gains access to the total state of the outer object. For example, in Java, the Iterator class would not need an explicit pointer to the LinkedList into which it points. In Java, static inner classes do not have this added pointer. They are the Java analog to nested classes in C++. 6.3.1 Use of an Inner Class to Access Object State The syntax for inner classes is rather complex. For that reason, we present a simple but somewhat artificial example to demonstrate the use of inner classes. We refactor the TimerTest example and extract a TalkingClock class. A talking clock is constructed with two parameters: the interval between announcements and a flag to turn beeps on or off. From the Library of Hristo Dimov Hristov 342 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes public class TalkingClock { private int interval; private boolean beep; public TalkingClock(int interval, boolean beep) { . . . } public void start() { . . . } public class TimePrinter implements ActionListener // an inner class { . . . } } Note that the TimePrinter class is now located inside the TalkingClock class. This does not mean that every TalkingClock has a TimePrinter instance field. As you will see, the TimePrinter objects are constructed by methods of the TalkingClock class. Here is the TimePrinter class in greater detail. Note that the actionPerformed method checks the beep flag before emitting a beep. public class TimePrinter implements ActionListener { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); } } Something surprising is going on. The TimePrinter class has no instance field or variable named beep. Instead, beep refers to the field of the TalkingClock object that created this TimePrinter. As you can see, an inner class method gets to access both its own data fields and those of the outer object creating it. For this to work, an object of an inner class always gets an implicit reference to the object that created it (see Figure 6.3). This reference is invisible in the definition of the inner class. However, to illuminate the concept, let us call the reference to the outer object outer. Then the actionPerformed method is equivalent to the following: public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (outer.beep) Toolkit.getDefaultToolkit().beep(); } From the Library of Hristo Dimov Hristov 6.3 Inner Classes 343 Figure 6.3 An inner class object has a reference to an outer class object. The outer class reference is set in the constructor. The compiler modifies all inner class constructors, adding a parameter for the outer class reference. The TimePrinter class defines no constructors; therefore, the compiler synthesizes a no-argument constructor, generating code like this: public TimePrinter(TalkingClock clock) // automatically generated code { outer = clock; } Again, please note that outer is not a Java keyword. We just use it to illustrate the mechanism involved in an inner class. When a TimePrinter object is constructed in the start method, the compiler passes the this reference to the current talking clock into the constructor: var listener = new TimePrinter(this); // parameter automatically added Listing 6.7 shows the complete program that tests the inner class. Have another look at the access control. Had the TimePrinter class been a regular class, it would have needed to access the beep flag through a public method of the TalkingClock class. Using an inner class is an improvement. There is no need to provide accessors that are of interest only to one other class. From the Library of Hristo Dimov Hristov 344 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes NOTE: We could have declared the TimePrinter class as private. Then only TalkingClock methods would be able to construct TimePrinter objects. Only inner classes can be private. Regular classes always have either package or public access. Listing 6.7 innerClass/InnerClassTest.java 1 package innerClass; 2 3 4 5 import java.awt.*; import java.awt.event.*; import java.time.*; 6 7 import javax.swing.*; 8 9 10 11 12 13 14 15 16 17 18 19 /** * This program demonstrates the use of inner classes. * @version 1.11 2017-12-14 * @author Cay Horstmann */ public class InnerClassTest { public static void main(String[] args) { var clock = new TalkingClock(1000, true); clock.start(); 20 // keep program running until the user selects "OK" JOptionPane.showMessageDialog(null, "Quit program?"); System.exit(0); 21 22 23 } 24 25 } 26 27 28 29 30 31 32 33 /** * A clock that prints the time in regular intervals. */ class TalkingClock { private int interval; private boolean beep; 34 35 36 37 38 39 /** * Constructs a talking clock * @param interval the interval between messages (in milliseconds) * @param beep true if the clock should beep */ From the Library of Hristo Dimov Hristov 6.3 Inner Classes 345 public TalkingClock(int interval, boolean beep) { this.interval = interval; this.beep = beep; } 40 41 42 43 44 45 /** * Starts the clock. */ public void start() { var listener = new TimePrinter(); var timer = new Timer(interval, listener); timer.start(); } 46 47 48 49 50 51 52 53 54 55 public class TimePrinter implements ActionListener { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); } } 56 57 58 59 60 61 62 63 64 65 } 6.3.2 Special Syntax Rules for Inner Classes In the preceding section, we explained the outer class reference of an inner class by calling it outer. Actually, the proper syntax for the outer reference is a bit more complex. The expression OuterClass.this denotes the outer class reference. For example, you can write the actionPerformed method of the TimePrinter inner class as public void actionPerformed(ActionEvent event) { . . . if (TalkingClock.this.beep) Toolkit.getDefaultToolkit().beep(); } Conversely, you can write the inner object constructor more explicitly, using the syntax outerObject.new InnerClass(construction parameters) For example: From the Library of Hristo Dimov Hristov 346 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes ActionListener listener = this.new TimePrinter(); Here, the outer class reference of the newly constructed TimePrinter object is set to the this reference of the method that creates the inner class object. This is the most common case. As always, the this. qualifier is redundant. However, it is also possible to set the outer class reference to another object by explicitly naming it. For example, since TimePrinter is a public inner class, you can construct a TimePrinter for any talking clock: var jabberer = new TalkingClock(1000, true); TalkingClock.TimePrinter listener = jabberer.new TimePrinter(); Note that you refer to an inner class as OuterClass.InnerClass when it occurs outside the scope of the outer class. NOTE: Any static fields declared in an inner class must be final and initialized with a compile-time constant. If the field was not a constant, it might not be unique. An inner class cannot have static methods. The Java Language Specification gives no reason for this limitation. It would have been possible to allow static methods that only access static fields and methods from the enclosing class. Apparently, the language designers decided that the complexities outweighed the benefits. 6.3.3 Are Inner Classes Useful? Actually Necessary? Secure? When inner classes were added to the Java language in Java 1.1, many programmers considered them a major new feature that was out of character with the Java philosophy of being simpler than C++. The inner class syntax is undeniably complex. (It gets more complex as we study anonymous inner classes later in this chapter.) It is not obvious how inner classes interact with other features of the language, such as access control and security. By adding a feature that was elegant and interesting rather than needed, has Java started down the road to ruin which has afflicted so many other languages? While we won’t try to answer this question completely, it is worth noting that inner classes are a phenomenon of the compiler, not the virtual machine. Inner classes are translated into regular class files with $ (dollar signs) delimiting outer and inner class names, and the virtual machine does not have any special knowledge about them. From the Library of Hristo Dimov Hristov 6.3 Inner Classes 347 For example, the TimePrinter class inside the TalkingClock class is translated to a class file TalkingClock$TimePrinter.class. To see this at work, try the following experiment: run the ReflectionTest program of Chapter 5, and give it the class TalkingClock$TimePrinter to reflect upon. Alternatively, simply use the javap utility: javap -private ClassName NOTE: If you use UNIX, remember to escape the $ character when you supply the class name on the command line. That is, run the ReflectionTest or javap program as java reflection.ReflectionTest innerClass.TalkingClock\$TimePrinter or javap -private innerClass.TalkingClock\$TimePrinter You will get the following printout: public class innerClass.TalkingClock$TimePrinter implements java.awt.event.ActionListener { final innerClass.TalkingClock this$0; public innerClass.TalkingClock$TimePrinter(innerClass.TalkingClock); public void actionPerformed(java.awt.event.ActionEvent); } You can plainly see that the compiler has generated an additional instance field, this$0, for the reference to the outer class. (The name this$0 is synthesized by the compiler—you cannot refer to it in your code.) You can also see the TalkingClock parameter for the constructor. If the compiler can automatically do this transformation, couldn’t you simply program the same mechanism by hand? Let’s try it. We would make TimePrinter a regular class, outside the TalkingClock class. When constructing a TimePrinter object, we pass it the this reference of the object that is creating it. class TalkingClock { . . . public void start() { var listener = new TimePrinter(this); var timer = new Timer(interval, listener); timer.start(); } } From the Library of Hristo Dimov Hristov 348 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes class TimePrinter implements ActionListener { private TalkingClock outer; . . . public TimePrinter(TalkingClock clock) { outer = clock; } } Now let us look at the actionPerformed method. It needs to access outer.beep. if (outer.beep) . . . // ERROR Here we run into a problem. The inner class can access the private data of the outer class, but our external TimePrinter class cannot. Thus, inner classes are genuinely more powerful than regular classes because they have more access privileges. You may well wonder how inner classes manage to acquire those added access privileges, if they are translated to regular classes with funny names—the virtual machine knows nothing at all about them. To solve this mystery, let’s again use the ReflectionTest program to spy on the TalkingClock class: class TalkingClock { private int interval; private boolean beep; public TalkingClock(int, boolean); static boolean access$0(TalkingClock); public void start(); } Notice the static access$0 method that the compiler added to the outer class. It returns the beep field of the object that is passed as a parameter. (The method name might be slightly different, such as access$000, depending on your compiler.) The inner class methods call that method. The statement if (beep) in the actionPerformed method of the TimePrinter class effectively makes the following call: if (TalkingClock.access$0(outer)) From the Library of Hristo Dimov Hristov 6.3 Inner Classes 349 Is this a security risk? You bet it is. It is an easy matter for someone else to invoke the access$0 method to read the private beep field. Of course, access$0 is not a legal name for a Java method. However, hackers who are familiar with the structure of class files can easily produce a class file with virtual machine instructions to call that method, for example, by using a hex editor. Since the secret methods have package access, the attack code would need to be placed inside the same package as the class under attack. To summarize, if an inner class accesses a private data field, then it is possible to access that data field through other classes added to the package of the outer class, but to do so requires skill and determination. A programmer cannot accidentally obtain access but must intentionally build or modify a class file for that purpose. NOTE: The synthesized constructors and methods can get quite convoluted. (Skip this note if you are squeamish.) Suppose we turn TimePrinter into a private inner class. There are no private classes in the virtual machine, so the compiler produces the next best thing, a class with package access and a private constructor: private TalkingClock$TimePrinter(TalkingClock); Of course, nobody can call that constructor, so there is a second constructor with package access: TalkingClock$TimePrinter(TalkingClock, TalkingClock$1); that calls the first one. The TalkingClock$1 class is synthesized solely to distinguish this constructor from others. The compiler translates the constructor call in the start method of the TalkingClock class to new TalkingClock$TimePrinter(this, null) 6.3.4 Local Inner Classes If you look carefully at the code of the TalkingClock example, you will find that you need the name of the type TimePrinter only once: when you create an object of that type in the start method. In a situation like this, you can define the class locally in a single method. public void start() { From the Library of Hristo Dimov Hristov 350 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes class TimePrinter implements ActionListener { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); } } var listener = new TimePrinter(); var timer = new Timer(interval, listener); timer.start(); } Local classes are never declared with an access specifier (that is, public or private). Their scope is always restricted to the block in which they are declared. Local classes have one great advantage: They are completely hidden from the outside world—not even other code in the TalkingClock class can access them. No method except start has any knowledge of the TimePrinter class. 6.3.5 Accessing Variables from Outer Methods Local classes have another advantage over other inner classes. Not only can they access the fields of their outer classes; they can even access local variables! However, those local variables must be effectively final. That means, they may never change once they have been assigned. Here is a typical example. Let’s move the interval and beep parameters from the TalkingClock constructor to the start method. public void start(int interval, boolean beep) { class TimePrinter implements ActionListener { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); } } var listener = new TimePrinter(); var timer = new Timer(interval, listener); timer.start(); } From the Library of Hristo Dimov Hristov 6.3 Inner Classes 351 Note that the TalkingClock class no longer needs to store a beep instance field. It simply refers to the beep parameter variable of the start method. Maybe this should not be so surprising. The line if (beep) . . . is, after all, ultimately inside the start method, so why shouldn’t it have access to the value of the beep variable? To see why there is a subtle issue here, let’s consider the flow of control more closely. 1. 2. The start method is called. The object variable listener is initialized by a call to the constructor of the inner class TimePrinter. The listener reference is passed to the Timer constructor, the timer is started, and the start method exits. At this point, the beep parameter variable of the start method no longer exists. A second later, the actionPerformed method executes if (beep) . . . 3. 4. For the code in the actionPerformed method to work, the TimePrinter class must have copied the beep field as a local variable of the start method, before the beep parameter value went away. That is indeed exactly what happens. In our example, the compiler synthesizes the name TalkingClock$1TimePrinter for the local inner class. If you use the ReflectionTest program again to spy on the TalkingClock$1TimePrinter class, you will get the following output: class TalkingClock$1TimePrinter { TalkingClock$1TimePrinter(TalkingClock, boolean); public void actionPerformed(java.awt.event.ActionEvent); final boolean val$beep; final TalkingClock this$0; } Note the boolean parameter to the constructor and the val$beep instance variable. When an object is created, the value beep is passed into the constructor and stored in the val$beep field. The compiler detects access of local variables, makes matching instance fields for each one, and copies the local variables into the constructor so that the instance fields can be initialized. From the Library of Hristo Dimov Hristov 352 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes 6.3.6 Anonymous Inner Classes When using local inner classes, you can often go a step further. If you want to make only a single object of this class, you don’t even need to give the class a name. Such a class is called an anonymous inner class. public void start(int interval, boolean beep) { var listener = new ActionListener() { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); } }; var timer = new Timer(interval, listener); timer.start(); } This syntax is very cryptic indeed. What it means is this: Create a new object of a class that implements the ActionListener interface, where the required method actionPerformed is the one defined inside the braces { }. In general, the syntax is new SuperType(construction parameters) { inner class methods and data } Here, SuperType can be an interface, such as ActionListener; then, the inner class implements that interface. SuperType can also be a class; then, the inner class extends that class. An anonymous inner class cannot have constructors because the name of a constructor must be the same as the name of a class, and the class has no name. Instead, the construction parameters are given to the superclass constructor. In particular, whenever an inner class implements an interface, it cannot have any construction parameters. Nevertheless, you must supply a set of parentheses as in new InterfaceType() { methods and data } From the Library of Hristo Dimov Hristov 6.3 Inner Classes 353 You have to look carefully to see the difference between the construction of a new object of a class and the construction of an object of an anonymous inner class extending that class. var queen = new Person("Mary"); // a Person object var count = new Person("Dracula") { . . . }; // an object of an inner class extending Person If the closing parenthesis of the construction parameter list is followed by an opening brace, then an anonymous inner class is being defined. NOTE: Even though an anonymous class cannot have constructors, you can provide an object initialization block: var count = new Person("Dracula") { { initialization } . . . }; Listing 6.8 contains the complete source code for the talking clock program with an anonymous inner class. If you compare this program with Listing 6.7, you will see that in this case, the solution with the anonymous inner class is quite a bit shorter and, hopefully, with some practice, as easy to comprehend. For many years, Java programmers routinely used anonymous inner classes for event listeners and other callbacks. Nowadays, you are better off using a lambda expression. For example, the start method from the beginning of this section can be written much more concisely with a lambda expression like this: public void start(int interval, boolean beep) { var timer = new Timer(interval, event -> { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); }); timer.start(); } From the Library of Hristo Dimov Hristov 354 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes NOTE: The following trick, called double brace initialization, takes advantage of the inner class syntax. Suppose you want to construct an array list and pass it to a method: var friends = new ArrayList<String>(); friends.add("Harry"); friends.add("Tony"); invite(friends); If you don’t need the array list again, it would be nice to make it anonymous. But then how can you add the elements? Here is how: invite(new ArrayList<String>() {{ add("Harry"); add("Tony"); }}); Note the double braces. The outer braces make an anonymous subclass of ArrayList. The inner braces are an object initialization block (see Chapter 4). In practice, this trick is rarely useful. More likely than not, the invite method is willing to accept any List<String>, and you can simply pass List.of("Harry", "Tony"). CAUTION: It is often convenient to make an anonymous subclass that is almost, but not quite, like its superclass. But you need to be careful with the equals method. In Chapter 5, we recommended that your equals methods use a test if (getClass() != other.getClass()) return false; An anonymous subclass will fail this test. TIP: When you produce logging or debugging messages, you often want to include the name of the current class, such as System.err.println("Something awful happened in " + getClass()); But that fails in a static method. After all, the call to getClass calls this.getClass(), and a static method has no this. Use the following expression instead: new Object(){}.getClass().getEnclosingClass() // gets class of static method Here, new Object(){} makes an anonymous object of an anonymous subclass of Object, and getEnclosingClass gets its enclosing class—that is, the class containing the static method. Listing 6.8 anonymousInnerClass/AnonymousInnerClassTest.java 1 package anonymousInnerClass; 2 From the Library of Hristo Dimov Hristov 6.3 3 4 5 Inner Classes 355 import java.awt.*; import java.awt.event.*; import java.time.*; 6 7 import javax.swing.*; 8 9 10 11 12 13 14 15 16 17 18 19 /** * This program demonstrates anonymous inner classes. * @version 1.12 2017-12-14 * @author Cay Horstmann */ public class AnonymousInnerClassTest { public static void main(String[] args) { var clock = new TalkingClock(); clock.start(1000, true); 20 // keep program running until the user selects "OK" JOptionPane.showMessageDialog(null, "Quit program?"); System.exit(0); 21 22 23 } 24 25 } 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 /** * A clock that prints the time in regular intervals. */ class TalkingClock { /** * Starts the clock. * @param interval the interval between messages (in milliseconds) * @param beep true if the clock should beep */ public void start(int interval, boolean beep) { var listener = new ActionListener() { public void actionPerformed(ActionEvent event) { System.out.println("At the tone, the time is " + Instant.ofEpochMilli(event.getWhen())); if (beep) Toolkit.getDefaultToolkit().beep(); } }; var timer = new Timer(interval, listener); timer.start(); } } From the Library of Hristo Dimov Hristov 356 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes 6.3.7 Static Inner Classes Occasionally, you may want to use an inner class simply to hide one class inside another—but you don’t need the inner class to have a reference to the outer class object. You can suppress the generation of that reference by declaring the inner class static. Here is a typical example of where you would want to do this. Consider the task of computing the minimum and maximum value in an array. Of course, you write one method to compute the minimum and another method to compute the maximum. When you call both methods, the array is traversed twice. It would be more efficient to traverse the array only once, computing both the minimum and the maximum simultaneously. double min = Double.POSITIVE_INFINITY; double max = Double.NEGATIVE_INFINITY; for (double v : values) { if (min > v) min = v; if (max < v) max = v; } However, the method must return two numbers. We can achieve that by defining a class Pair that holds two values: class Pair { private double first; private double second; public Pair(double f, double s) { first = f; second = s; } public double getFirst() { return first; } public double getSecond() { return second; } } The minmax method can then return an object of type Pair. class ArrayAlg { public static Pair minmax(double[] values) { . . . return new Pair(min, max); } } From the Library of Hristo Dimov Hristov 6.3 Inner Classes 357 The caller of the method uses the getFirst and getSecond methods to retrieve the answers: Pair p = ArrayAlg.minmax(d); System.out.println("min = " + p.getFirst()); System.out.println("max = " + p.getSecond()); Of course, the name Pair is an exceedingly common name, and in a large project, it is quite possible that some other programmer had the same bright idea—but made a Pair class that contains a pair of strings. We can solve this potential name clash by making Pair a public inner class inside ArrayAlg. Then the class will be known to the public as ArrayAlg.Pair: ArrayAlg.Pair p = ArrayAlg.minmax(d); However, unlike the inner classes that we used in previous examples, we do not want to have a reference to any other object inside a Pair object. That reference can be suppressed by declaring the inner class static: class ArrayAlg { public static class Pair { . . . } . . . } Of course, only inner classes can be declared static. A static inner class is exactly like any other inner class, except that an object of a static inner class does not have a reference to the outer class object that generated it. In our example, we must use a static inner class because the inner class object is constructed inside a static method: public static Pair minmax(double[] d) { . . . return new Pair(min, max); } Had the Pair class not been declared as static, the compiler would have complained that there was no implicit object of type ArrayAlg available to initialize the inner class object. NOTE: Use a static inner class whenever the inner class does not need to access an outer class object. Some programmers use the term nested class to describe static inner classes. From the Library of Hristo Dimov Hristov 358 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes NOTE: Unlike regular inner classes, static inner classes can have static fields and methods. NOTE: Inner classes that are declared inside an interface are automatically static and public. Listing 6.9 contains the complete source code of the ArrayAlg class and the nested Pair class. Listing 6.9 staticInnerClass/StaticInnerClassTest.java 1 package staticInnerClass; 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 /** * This program demonstrates the use of static inner classes. * @version 1.02 2015-05-12 * @author Cay Horstmann */ public class StaticInnerClassTest { public static void main(String[] args) { var values = new double[20]; for (int i = 0; i < values.length; i++) values[i] = 100 * Math.random(); ArrayAlg.Pair p = ArrayAlg.minmax(values); System.out.println("min = " + p.getFirst()); System.out.println("max = " + p.getSecond()); } } 20 21 22 23 24 25 26 27 28 29 class ArrayAlg { /** * A pair of floating-point numbers */ public static class Pair { private double first; private double second; 30 31 32 /** * Constructs a pair from two floating-point numbers From the Library of Hristo Dimov Hristov 6.3 Inner Classes 359 * @param f the first number * @param s the second number */ public Pair(double f, double s) { first = f; second = s; } 33 34 35 36 37 38 39 40 41 /** * Returns the first number of the pair * @return the first number */ public double getFirst() { return first; } 42 43 44 45 46 47 48 49 50 /** * Returns the second number of the pair * @return the second number */ public double getSecond() { return second; } 51 52 53 54 55 56 57 58 } 59 60 /** * Computes both the minimum and the maximum of an array * @param values an array of floating-point numbers * @return a pair whose first element is the minimum and whose second element * is the maximum */ public static Pair minmax(double[] values) { double min = Double.POSITIVE_INFINITY; double max = Double.NEGATIVE_INFINITY; for (double v : values) { if (min > v) min = v; if (max < v) max = v; } return new Pair(min, max); } 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 } From the Library of Hristo Dimov Hristov 360 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes 6.4 Service Loaders Sometimes, you develop an application with a service architecture. There are platforms that encourage this approach, such as OSGi (http://osgi.org), which are used in development environments, application servers, and other complex applications. Such platforms go well beyond the scope of this book, but the JDK also offers a simple mechanism for loading services, which we describe here. This mechanism is well supported by the Java Platform Module System— see Chapter 9 of Volume II. Often, when providing a service, a program wants to give the service designer some freedom of how to implement the service’s features. It can also be desirable to have multiple implementations to choose from. The ServiceLoader class makes it easy to load services that conform to a common interface. Define an interface (or, if you prefer, a superclass) with the methods that each instance of the service should provide. For example, suppose your service provides encryption. package serviceLoader; public interface Cipher { byte[] encrypt(byte[] source, byte[] key); byte[] decrypt(byte[] source, byte[] key); int strength(); } The service provider supplies one or more classes that implement this service, for example package serviceLoader.impl; public class CaesarCipher implements Cipher { public byte[] encrypt(byte[] source, byte[] key) { var result = new byte[source.length]; for (int i = 0; i < source.length; i++) result[i] = (byte)(source[i] + key[0]); return result; } public byte[] decrypt(byte[] source, byte[] key) { return encrypt(source, new byte[] { (byte) -key[0] }); } From the Library of Hristo Dimov Hristov 6.4 Service Loaders 361 public int strength() { return 1; } } The implementing classes can be in any package, not necessarily the same package as the service interface. Each of them must have a no-argument constructor. Now add the names of the classes to a UTF-8 encoded text file in a file in the META-INF/services directory whose name matches the fully qualified interface name. In our example, the file META-INF/services/serviceLoader.Cipher would contain the line serviceLoader.impl.CaesarCipher In this example, we provide a single implementing class. You could also provide multiple classes and later pick among them. With this preparation done, the program initializes a service loader as follows: public static ServiceLoader<Cipher> cipherLoader = ServiceLoader.load(Cipher.class); This should be done just once in the program. The iterator method of the service loader returns an iterator through all provided implementations of the service. (See Chapter 9 for more information about iterators.) It is easiest to use an enhanced for loop to traverse them. In the loop, pick an appropriate object to carry out the service. public static Cipher getCipher(int minStrength) { for (Cipher cipher : cipherLoader) // implicitly calls cipherLoader.iterator() { if (cipher.strength() >= minStrength) return cipher; } return null; } Alternatively, you can use streams (see Chapter 1 of Volume II) to locate the desired service. The stream method yields a stream of ServiceLoader.Provider instances. That interface has methods type and get for getting the provider class and the provider instance. If you select a provider by type, then you just call type and no service instances are unnecessarily instantiated. public static Optional<Cipher> getCipher2(int minStrength) { return cipherLoader.stream() .filter(descr -> descr.type() == serviceLoader.impl.CaesarCipher.class) .findFirst() .map(ServiceLoader.Provider::get); } From the Library of Hristo Dimov Hristov 362 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes Finally, if you are willing to take any service instance, simply call findFirst: Optional<Cipher> cipher = cipherLoader.findFirst(); The Optional class is explained in Chapter 1 of Volume II. java.util.ServiceLoader<S> 1.6 • static <S> ServiceLoader<S> load(Class<S> service) creates a service loader for loading the classes that implement the given service interface. • Iterator<S> iterator() yields an iterator that lazily loads the service classes. That is, a class is loaded whenever the iterator advances. • Stream<ServiceLoader.Provider<S>> stream() 9 returns a stream of provider descriptors, so that a provider of a desired class can be loaded lazily. • Optional<S> findFirst() 9 finds the first available service provider, if any. java.util.ServiceLoader.Provider<S> 9 • Class<? extends S> type() gets the type of this provider. • S get() gets an instance of this provider. 6.5 Proxies In the final section of this chapter, we discuss proxies. You can use a proxy to create, at runtime, new classes that implement a given set of interfaces. Proxies are only necessary when you don’t yet know at compile time which interfaces you need to implement. This is not a common situation for application programmers, so feel free to skip this section if you are not interested in advanced wizardry. However, for certain systems programming applications, the flexibility that proxies offer can be very important. From the Library of Hristo Dimov Hristov 6.5 Proxies 363 6.5.1 When to Use Proxies Suppose you want to construct an object of a class that implements one or more interfaces whose exact nature you may not know at compile time. This is a difficult problem. To construct an actual class, you can simply use the newInstance method or use reflection to find a constructor. But you can’t instantiate an interface. You need to define a new class in a running program. To overcome this problem, some programs generate code, place it into a file, invoke the compiler, and then load the resulting class file. Naturally, this is slow, and it also requires deployment of the compiler together with the program. The proxy mechanism is a better solution. The proxy class can create brand-new classes at runtime. Such a proxy class implements the interfaces that you specify. In particular, the proxy class has the following methods: • All methods required by the specified interfaces; and • All methods defined in the Object class (toString, equals, and so on). However, you cannot define new code for these methods at runtime. Instead, you must supply an invocation handler. An invocation handler is an object of any class that implements the InvocationHandler interface. That interface has a single method: Object invoke(Object proxy, Method method, Object[] args) Whenever a method is called on the proxy object, the invoke method of the invocation handler gets called, with the Method object and parameters of the original call. The invocation handler must then figure out how to handle the call. 6.5.2 Creating Proxy Objects To create a proxy object, use the newProxyInstance method of the Proxy class. The method has three parameters: • A class loader. As part of the Java security model, different class loaders can be used for platform and application classes, classes that are downloaded from the Internet, and so on. We will discuss class loaders in Chapter 9 of Volume II. In this example, we specify the “system class loader” that loads platform and application classes. • An array of Class objects, one for each interface to be implemented. • An invocation handler. There are two remaining questions. How do we define the handler? And what can we do with the resulting proxy object? The answers depend, of course, From the Library of Hristo Dimov Hristov 364 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes on the problem that we want to solve with the proxy mechanism. Proxies can be used for many purposes, such as • Routing method calls to remote servers • Associating user interface events with actions in a running program • Tracing method calls for debugging purposes In our example program, we use proxies and invocation handlers to trace method calls. We define a TraceHandler wrapper class that stores a wrapped object. Its invoke method simply prints the name and parameters of the method to be called and then calls the method with the wrapped object as the implicit parameter. class TraceHandler implements InvocationHandler { private Object target; public TraceHandler(Object t) { target = t; } public Object invoke(Object proxy, Method m, Object[] args) throws Throwable { // print method name and parameters . . . // invoke actual method return m.invoke(target, args); } } Here is how you construct a proxy object that causes the tracing behavior whenever one of its methods is called: Object value = . . .; // construct wrapper var handler = new TraceHandler(value); // construct proxy for one or more interfaces var interfaces = new Class[] { Comparable.class}; Object proxy = Proxy.newProxyInstance( ClassLoader.getSystemClassLoader(), new Class[] { Comparable.class } , handler); Now, whenever a method from one of the interfaces is called on proxy, the method name and parameters are printed out and the method is then invoked on value. From the Library of Hristo Dimov Hristov 6.5 Proxies 365 In the program shown in Listing 6.10, we use proxy objects to trace a binary search. We fill an array with proxies to the integers 1 . . . 1000. Then we invoke the binarySearch method of the Arrays class to search for a random integer in the array. Finally, we print the matching element. var elements = new Object[1000]; // fill elements with proxies for the integers 1 . . . 1000 for (int i = 0; i < elements.length; i++) { Integer value = i + 1; elements[i] = Proxy.newProxyInstance(. . .); // proxy for value; } // construct a random integer Integer key = new Random().nextInt(elements.length) + 1; // search for the key int result = Arrays.binarySearch(elements, key); // print match if found if (result >= 0) System.out.println(elements[result]); The Integer class implements the Comparable interface. The proxy objects belong to a class that is defined at runtime. (It has a name such as $Proxy0.) That class also implements the Comparable interface. However, its compareTo method calls the invoke method of the proxy object’s handler. NOTE: As you saw earlier in this chapter, the Integer class actually implements Comparable<Integer>. However, at runtime, all generic types are erased and the proxy is constructed with the class object for the raw Comparable class. The binarySearch method makes calls like this: if (elements[i].compareTo(key) < 0) . . . Since we filled the array with proxy objects, the compareTo calls the invoke method of the TraceHandler class. That method prints the method name and parameters and then invokes compareTo on the wrapped Integer object. Finally, at the end of the sample program, we call System.out.println(elements[result]); The println method calls toString on the proxy object, and that call is also redirected to the invocation handler. Here is the complete trace of a program run: From the Library of Hristo Dimov Hristov 366 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes 500.compareTo(288) 250.compareTo(288) 375.compareTo(288) 312.compareTo(288) 281.compareTo(288) 296.compareTo(288) 288.compareTo(288) 288.toString() You can see how the binary search algorithm homes in on the key by cutting the search interval in half in every step. Note that the toString method is proxied even though it does not belong to the Comparable interface—as you will see in the next section, certain Object methods are always proxied. Listing 6.10 proxy/ProxyTest.java 1 package proxy; 2 3 4 import java.lang.reflect.*; import java.util.*; 5 6 7 8 9 10 11 12 13 14 15 /** * This program demonstrates the use of proxies. * @version 1.01 2018-04-10 * @author Cay Horstmann */ public class ProxyTest { public static void main(String[] args) { var elements = new Object[1000]; 16 17 18 19 20 21 22 23 24 25 26 // fill elements with proxies for the integers 1 . . . 1000 for (int i = 0; i < elements.length; i++) { Integer value = i + 1; var handler = new TraceHandler(value); Object proxy = Proxy.newProxyInstance( ClassLoader.getSystemClassLoader(), new Class[] { Comparable.class } , handler); elements[i] = proxy; } 27 28 29 // construct a random integer Integer key = new Random().nextInt(elements.length) + 1; 30 From the Library of Hristo Dimov Hristov 6.5 Proxies 367 // search for the key int result = Arrays.binarySearch(elements, key); 31 32 33 // print match if found if (result >= 0) System.out.println(elements[result]); 34 35 } 36 37 } 38 39 40 41 42 43 44 45 /** * An invocation handler that prints out the method name and parameters, then * invokes the original method */ class TraceHandler implements InvocationHandler { private Object target; 46 /** * Constructs a TraceHandler * @param t the implicit parameter of the method call */ public TraceHandler(Object t) { target = t; } 47 48 49 50 51 52 53 54 55 public Object invoke(Object proxy, Method m, Object[] args) throws Throwable { // print implicit argument System.out.print(target); // print method name System.out.print("." + m.getName() + "("); // print explicit arguments if (args != null) { for (int i = 0; i < args.length; i++) { System.out.print(args[i]); if (i < args.length - 1) System.out.print(", "); } } System.out.println(")"); 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 // invoke actual method return m.invoke(target, args); 73 74 } 75 76 } From the Library of Hristo Dimov Hristov 368 Chapter 6 Interfaces, Lambda Expressions, and Inner Classes 6.5.3 Properties of Proxy Classes Now that you have seen proxy classes in action, let’s go over some of their properties. Remember that proxy classes are created on the fly in a running program. However, once they are created, they are regular classes, just like any other classes in the virtual machine. All proxy classes extend the class Proxy. A proxy class has only one instance field—the invocation handler, which is defined in the Proxy superclass. Any additional data required to carry out the proxy objects’ tasks must be stored in the invocation handler. For example, when we proxied Comparable objects in the program shown in Listing 6.10, the TraceHandler wrapped the actual objects. All proxy classes override the toString, equals, and hashCode methods of the Object class. Like all proxy methods, these methods simply call invoke on the invocation handler. The other methods of the Object class (such as clone and getClass) are not redefined. The names of proxy classes are not defined. The Proxy class in Oracle’s virtual machine generates class names that begin with the string $Proxy. There is only one proxy class for a particular class loader and ordered set of interfaces. That is, if you call the newProxyInstance method twice with the same class loader and interface array, you get two objects of the same class. You can also obtain that class with the getProxyClass method: Class proxyClass = Proxy.getProxyClass(null, interfaces); A proxy class is always public and final. If all interfaces that the proxy class implements are public, the proxy class does not belong to any particular package. Otherwise, all non-public interfaces must belong to the same package, and the proxy class will also belong to that package. You can test whether a particular Class object represents a proxy class by calling the isProxyClass method of the Proxy class. java.lang.reflect.InvocationHandler 1.3 • Object invoke(Object proxy, Method method, Object[] args) define this method to contain the action that you want carried out whenever a method was invoked on the proxy object. From the Library of Hristo Dimov Hristov 6.5 Proxies 369 java.lang.reflect.Proxy 1.3 • static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces) returns the proxy class that implements the given interfaces. • static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler handler) constructs a new instance of the proxy class that implements the given interfaces. All methods call the invoke method of the given handler object. • static boolean isProxyClass(Class<?> cl) returns true if cl is a proxy class. This ends our final chapter on the object-oriented features of the Java programming language. Interfaces, lambda expressions, and inner classes are concepts that you will encounter frequently, whereas cloning, service loaders, and proxies are advanced techniques that are of interest mainly to library designers and tool builders, not application programmers. You are now ready to learn how to deal with exceptional situations in your programs in Chapter 7. From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov CHAPTER 7 Exceptions, Assertions, and Logging In this chapter • 7.1 Dealing with Errors, page 372 • 7.2 Catching Exceptions, page 381 • 7.3 Tips for Using Exceptions, page 396 • 7.4 Using Assertions, page 399 • 7.5 Logging, page 403 • 7.6 Debugging Tips, page 425 In a perfect world, users would never enter data in the wrong form, files they choose to open would always exist, and code would never have bugs. So far, we have mostly presented code as if we lived in this kind of perfect world. It is now time to turn to the mechanisms the Java programming language has for dealing with the real world of bad data and buggy code. Encountering errors is unpleasant. If a user loses all the work he or she did during a program session because of a programming mistake or some external circumstance, that user may forever turn away from your program. At the very least, you must: • Notify the user of an error; 371 From the Library of Hristo Dimov Hristov 372 Chapter 7 Exceptions, Assertions, and Logging • Save all work; and • Allow users to gracefully exit the program. For exceptional situations, such as bad input data with the potential to bomb the program, Java uses a form of error trapping called, naturally enough, exception handling. Exception handling in Java is similar to that in C++ or Delphi. The first part of this chapter covers Java’s exceptions. During testing, you need to run lots of checks to make sure your program does the right thing. But those checks can be time-consuming and unnecessary after testing has completed. You could just remove the checks and stick them back in when additional testing is required—but that is tedious. The second part of this chapter shows you how to use the assertion facility for selectively activating checks. When your program does the wrong thing, you can’t always communicate with the user or terminate. Instead, you may want to record the problem for later analysis. The third part of this chapter discusses the standard Java logging framework. 7.1 Dealing with Errors Suppose an error occurs while a Java program is running. The error might be caused by a file containing wrong information, a flaky network connection, or (we hate to mention it) use of an invalid array index or an object reference that hasn’t yet been assigned to an object. Users expect that programs will act sensibly when errors happen. If an operation cannot be completed because of an error, the program ought to either • Return to a safe state and enable the user to execute other commands; or • Allow the user to save all work and terminate the program gracefully. This may not be easy to do, because the code that detects (or even causes) the error condition is usually far removed from the code that can roll back the data to a safe state or save the user’s work and exit cheerfully. The mission of exception handling is to transfer control from where the error occurred to an error handler that can deal with the situation. To handle exceptional situations in your program, you must take into account the errors and problems that may occur. What sorts of problems do you need to consider? • User input errors. In addition to the inevitable typos, some users like to blaze their own trail instead of following directions. Suppose, for example, that a user asks to connect to a URL that is syntactically wrong. Your code From the Library of Hristo Dimov Hristov 7.1 Dealing with Errors 373 should check the syntax, but suppose it does not. Then the network layer will complain. • Device errors. Hardware does not always do what you want it to. The printer may be turned off. A web page may be temporarily unavailable. Devices will often fail in the middle of a task. For example, a printer may run out of paper during printing. • Physical limitations. Disks can fill up; you can run out of available memory. • Code errors. A method may not perform correctly. For example, it could deliver wrong answers or use other methods incorrectly. Computing an invalid array index, trying to find a nonexistent entry in a hash table, or trying to pop an empty stack are all examples of a code error. The traditional reaction to an error in a method is to return a special error code that the calling method analyzes. For example, methods that read information back from files often return a -1 end-of-file value marker rather than a standard character. This can be an efficient method for dealing with many exceptional conditions. Another common return value to denote an error condition is the null reference. Unfortunately, it is not always possible to return an error code. There may be no obvious way of distinguishing valid and invalid data. A method returning an integer cannot simply return -1 to denote the error; the value -1 might be a perfectly valid result. Instead, as we mentioned back in Chapter 5, Java allows every method an alternative exit path if it is unable to complete its task in the normal way. In this situation, the method does not return a value. Instead, it throws an object that encapsulates the error information. Note that the method exits immediately; it does not return its normal (or any) value. Moreover, execution does not resume at the code that called the method; instead, the exception-handling mechanism begins its search for an exception handler that can deal with this particular error condition. Exceptions have their own syntax and are part of a special inheritance hierarchy. We’ll take up the syntax first and then give a few hints on how to use this language feature effectively. 7.1.1 The Classification of Exceptions In the Java programming language, an exception object is always an instance of a class derived from Throwable. As you will soon see, you can create your own exception classes if those built into Java do not suit your needs. Figure 7.1 is a simplified diagram of the exception hierarchy in Java. From the Library of Hristo Dimov Hristov 374 Chapter 7 Exceptions, Assertions, and Logging Figure 7.1 Exception hierarchy in Java Notice that all exceptions descend from Throwable, but the hierarchy immediately splits into two branches: Error and Exception. The Error hierarchy describes internal errors and resource exhaustion situations inside the Java runtime system. You should not throw an object of this type. There is little you can do if such an internal error occurs, beyond notifying the user and trying to terminate the program gracefully. These situations are quite rare. When doing Java programming, focus on the Exception hierarchy. The Exception hierarchy also splits into two branches: exceptions that derive from RuntimeException and those that do not. The general rule is this: A RuntimeException happens because you made a programming error. Any other exception occurs because a bad thing, such as an I/O error, happened to your otherwise good program. Exceptions that inherit from RuntimeException include such problems as • A bad cast • An out-of-bounds array access • A null pointer access From the Library of Hristo Dimov Hristov 7.1 Dealing with Errors 375 Exceptions that do not inherit from RuntimeException include • Trying to read past the end of a file • Trying to open a file that doesn’t exist • Trying to find a Class object for a string that does not denote an existing class The rule “If it is a RuntimeException, it was your fault” works pretty well. You could have avoided that ArrayIndexOutOfBoundsException by testing the array index against the array bounds. The NullPointerException would not have happened had you checked whether the variable was null before using it. How about a file that doesn’t exist? Can’t you first check whether the file exists, and then open it? Well, the file might be deleted right after you checked for its existence. Thus, the notion of “existence” depends on the environment, not just on your code. The Java Language Specification calls any exception that derives from the class Error or the class RuntimeException an unchecked exception. All other exceptions are called checked exceptions. This is useful terminology that we also adopt. The compiler checks that you provide exception handlers for all checked exceptions. NOTE: The name RuntimeException is somewhat confusing. Of course, all of the errors we are discussing occur at runtime. C++ NOTE: If you are familiar with the (much more limited) exception hierarchy of the standard C++ library, you may be really confused at this point. C++ has two fundamental exception classes, runtime_error and logic_error. The logic_error class is the equivalent of Java’s RuntimeException and also denotes logical errors in the program. The runtime_error class is the base class for exceptions caused by unpredictable problems. It is equivalent to those exceptions in Java that are not of type RuntimeException. 7.1.2 Declaring Checked Exceptions A Java method can throw an exception if it encounters a situation it cannot handle. The idea is simple: A method will not only tell the Java compiler what values it can return, it is also going to tell the compiler what can go wrong. For example, code that attempts to read from a file knows that the file might not exist or that it might be empty. The code that tries to process the From the Library of Hristo Dimov Hristov 376 Chapter 7 Exceptions, Assertions, and Logging information in a file therefore will need to notify the compiler that it can throw some sort of IOException. The place in which you advertise that your method can throw an exception is the header of the method; the header changes to reflect the checked exceptions the method can throw. For example, here is the declaration of one of the constructors of the FileInputStream class from the standard library. (See Chapter 1 of Volume II for more on input and output.) public FileInputStream(String name) throws FileNotFoundException The declaration says that this constructor produces a FileInputStream object from a String parameter but that it also can go wrong in a special way—by throwing a FileNotFoundException. If this sad state should come to pass, the constructor call will not initialize a new FileInputStream object but instead will throw an object of the FileNotFoundException class. If it does, the runtime system will begin to search for an exception handler that knows how to deal with FileNotFoundException objects. When you write your own methods, you don’t have to advertise every possible throwable object that your method might actually throw. To understand when (and what) you have to advertise in the throws clause of the methods you write, keep in mind that an exception is thrown in any of the following four situations: • You call a method that throws a checked exception—for example, the FileInputStream constructor. • You detect an error and throw a checked exception with the throw statement (we cover the throw statement in the next section). • You make a programming error, such as a[-1] = 0 that gives rise to an unchecked exception (in this case, an ArrayIndexOutOfBoundsException). • An internal error occurs in the virtual machine or runtime library. If either of the first two scenarios occurs, you must tell the programmers who will use your method about the possibility of an exception. Why? Any method that throws an exception is a potential death trap. If no handler catches the exception, the current thread of execution terminates. As with Java methods that are part of the supplied classes, you declare that your method may throw an exception with an exception specification in the method header. class MyAnimation { . . . From the Library of Hristo Dimov Hristov 7.1 Dealing with Errors 377 public Image loadImage(String s) throws IOException { . . . } } If a method might throw more than one checked exception type, you must list all exception classes in the header. Separate them by commas, as in the following example: class MyAnimation { . . . public Image loadImage(String s) throws FileNotFoundException, EOFException { . . . } } However, you do not need to advertise internal Java errors—that is, exceptions inheriting from Error. Any code could potentially throw those exceptions, and they are entirely beyond your control. Similarly, you should not advertise unchecked exceptions inheriting from RuntimeException. class MyAnimation { . . . void drawImage(int i) throws ArrayIndexOutOfBoundsException // bad style { . . . } } These runtime errors are completely under your control. If you are so concerned about array index errors, you should spend your time fixing them instead of advertising the possibility that they can happen. In summary, a method must declare all the checked exceptions that it might throw. Unchecked exceptions are either beyond your control (Error) or result from conditions that you should not have allowed in the first place (RuntimeException). If your method fails to faithfully declare all checked exceptions, the compiler will issue an error message. Of course, as you have already seen in quite a few examples, instead of declaring the exception, you can also catch it. Then the exception won’t be thrown out of the method, and no throws specification is necessary. You will From the Library of Hristo Dimov Hristov 378 Chapter 7 Exceptions, Assertions, and Logging see later in this chapter how to decide whether to catch an exception or to enable someone else to catch it. CAUTION: If you override a method from a superclass, the checked exceptions that the subclass method declares cannot be more general than those of the superclass method. (It is OK to throw more specific exceptions, or not to throw any exceptions in the subclass method.) In particular, if the superclass method throws no checked exception at all, neither can the subclass. For example, if you override JComponent.paintComponent, your paintComponent method must not throw any checked exceptions, because the superclass method doesn’t throw any. When a method in a class declares that it throws an exception that is an instance of a particular class, it may throw an exception of that class or of any of its subclasses. For example, the FileInputStream constructor could have declared that it throws an IOException. In that case, you would not have known what kind of IOException it is; it could be a plain IOException or an object of one of the various subclasses, such as FileNotFoundException. C++ NOTE: The throws specifier is the same as the throw specifier in C++, with one important difference. In C++, throw specifiers are enforced at runtime, not at compile time. That is, the C++ compiler pays no attention to exception specifications. But if an exception is thrown in a function that is not part of the throw list, the unexpected function is called, and, by default, the program terminates. Also, in C++, a function may throw any exception if no throw specification is given. In Java, a method without a throws specifier may not throw any checked exceptions at all. 7.1.3 How to Throw an Exception Now, suppose something terrible has happened in your code. You have a method, readData, that is reading in a file whose header promised Content-length: 1024 but you got an end of file after 733 characters. You may decide this situation is so abnormal that you want to throw an exception. You need to decide what exception type to throw. Some kind of IOException would be a good choice. Perusing the Java API documentation, you find an EOFException with the description “Signals that an EOF has been reached unexpectedly during input.” Perfect. Here is how you throw it: From the Library of Hristo Dimov Hristov 7.1 Dealing with Errors 379 throw new EOFException(); or, if you prefer, var e = new EOFException(); throw e; Here is how it all fits together: String readData(Scanner in) throws EOFException { . . . while (. . .) { if (!in.hasNext()) // EOF encountered { if (n < len) throw new EOFException(); } . . . } return s; } The EOFException has a second constructor that takes a string argument. You can put this to good use by describing the exceptional condition more carefully. String gripe = "Content-length: " + len + ", Received: " + n; throw new EOFException(gripe); As you can see, throwing an exception is easy if one of the existing exception classes works for you. In this case: 1. 2. 3. Find an appropriate exception class. Make an object of that class. Throw it. Once a method throws an exception, it does not return to its caller. This means you do not have to worry about cooking up a default return value or an error code. C++ NOTE: Throwing an exception is the same in C++ and in Java, with one small difference. In Java, you can throw only objects of subclasses of Throwable. In C++, you can throw values of any type. From the Library of Hristo Dimov Hristov 380 Chapter 7 Exceptions, Assertions, and Logging 7.1.4 Creating Exception Classes Your code may run into a problem which is not adequately described by any of the standard exception classes. In this case, it is easy enough to create your own exception class. Just derive it from Exception, or from a child class of Exception such as IOException. It is customary to give both a default constructor and a constructor that contains a detailed message. (The toString method of the Throwable superclass returns a string containing that detailed message, which is handy for debugging.) class FileFormatException extends IOException { public FileFormatException() {} public FileFormatException(String gripe) { super(gripe); } } Now you are ready to throw your very own exception type. String readData(Scanner in) throws FileFormatException { . . . while (. . .) { if (ch == -1) // EOF encountered { if (n < len) throw new FileFormatException(); } . . . } return s; } java.lang.Throwable 1.0 • Throwable() constructs a new Throwable object with no detailed message. • Throwable(String message) constructs a new Throwable object with the specified detailed message. By convention, all derived exception classes support both a default constructor and a constructor with a detailed message. • String getMessage() gets the detailed message of the Throwable object. From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 381 7.2 Catching Exceptions You now know how to throw an exception. It is pretty easy: You throw it and you forget it. Of course, some code has to catch the exception. Catching exceptions requires more planning. That’s what the next sections will cover. 7.2.1 Catching an Exception If an exception occurs that is not caught anywhere, the program will terminate and print a message to the console, giving the type of the exception and a stack trace. GUI programs (both applets and applications) catch exceptions, print stack trace messages, and then go back to the user interface processing loop. (When you are debugging a GUI program, it is a good idea to keep the console on the screen and not minimized.) To catch an exception, set up a try/catch block. The simplest form of the try block is as follows: try { code more code more code } catch (ExceptionType e) { handler for this type } If any code inside the try block throws an exception of the class specified in the catch clause, then 1. 2. The program skips the remainder of the code in the try block. The program executes the handler code inside the catch clause. If none of the code inside the try block throws an exception, then the program skips the catch clause. If any of the code in a method throws an exception of a type other than the one named in the catch clause, this method exits immediately. (Hopefully, one of its callers has already provided a catch clause for that type.) To show this at work, here’s some fairly typical code for reading in data: public void read(String filename) { try { From the Library of Hristo Dimov Hristov 382 Chapter 7 Exceptions, Assertions, and Logging var in = new FileInputStream(filename); int b; while ((b = in.read()) != -1) { process input } } catch (IOException exception) { exception.printStackTrace(); } } Notice that most of the code in the try clause is straightforward: It reads and processes bytes until we encounter the end of the file. As you can see by looking at the Java API, there is the possibility that the read method will throw an IOException. In that case, we skip out of the entire while loop, enter the catch clause, and generate a stack trace. For a toy program, that seems like a reasonable way to deal with this exception. What other choice do you have? Often, the best choice is to do nothing at all and simply pass the exception on to the caller. If an error occurs in the read method, let the caller of the read method worry about it! If we take that approach, then we have to advertise the fact that the method may throw an IOException. public void read(String filename) throws IOException { var in = new FileInputStream(filename); int b; while ((b = in.read()) != -1) { process input } } Remember, the compiler strictly enforces the throws specifiers. If you call a method that throws a checked exception, you must either handle it or pass it on. Which of the two is better? As a general rule, you should catch those exceptions that you know how to handle and propagate those that you do not know how to handle. When you propagate an exception, you must add a throws specifier to alert the caller that an exception may be thrown. Look at the Java API documentation to see what methods throw which exceptions. Then decide whether you should handle them or add them to the throws From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 383 list. There is nothing embarrassing about the latter choice. It is better to direct an exception to a competent handler than to squelch it. Please keep in mind that there is, as we mentioned earlier, one exception to this rule. If you are writing a method that overrides a superclass method which throws no exceptions (such as paintComponent in JComponent), then you must catch each checked exception in your method’s code. You are not allowed to add more throws specifiers to a subclass method than are present in the superclass method. C++ NOTE: Catching exceptions is almost the same in Java and in C++. Strictly speaking, the analog of catch (Exception e) // Java is catch (Exception& e) // C++ There is no analog to the C++ catch (. . .). This is not needed in Java because all exceptions derive from a common superclass. 7.2.2 Catching Multiple Exceptions You can catch multiple exception types in a try block and handle each type differently. Use a separate catch clause for each type, as in the following example: try { code that might throw exceptions } catch (FileNotFoundException e) { emergency action for missing files } catch (UnknownHostException e) { emergency action for unknown hosts } catch (IOException e) { emergency action for all other I/O problems } The exception object may contain information about the nature of the exception. To find out more about the object, try From the Library of Hristo Dimov Hristov 384 Chapter 7 Exceptions, Assertions, and Logging e.getMessage() to get the detailed error message (if there is one), or e.getClass().getName() to get the actual type of the exception object. As of Java 7, you can catch multiple exception types in the same catch clause. For example, suppose that the action for missing files and unknown hosts is the same. Then you can combine the catch clauses: try { code that might throw exceptions } catch (FileNotFoundException | UnknownHostException e) { emergency action for missing files and unknown hosts } catch (IOException e) { emergency action for all other I/O problems } This feature is only needed when catching exception types that are not subclasses of one another. NOTE: When you catch multiple exceptions, the exception variable is implicitly final. For example, you cannot assign a different value to e in the body of the clause catch (FileNotFoundException | UnknownHostException e) { . . . } NOTE: Catching multiple exceptions doesn’t just make your code look simpler but also more efficient. The generated bytecodes contain a single block for the shared catch clause. 7.2.3 Rethrowing and Chaining Exceptions You can throw an exception in a catch clause. Typically, you do this when you want to change the exception type. If you build a subsystem that other programmers use, it makes a lot of sense to use an exception type that indicates a failure of the subsystem. An example of such an exception type is the ServletException. The code that executes a servlet may not want to know in From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 385 minute detail what went wrong, but it definitely wants to know that the servlet was at fault. Here is how you can catch an exception and rethrow it: try { access the database } catch (SQLException e) { throw new ServletException("database error: " + e.getMessage()); } Here, the ServletException is constructed with the message text of the exception. However, it is a better idea to set the original exception as the “cause” of the new exception: try { access the database } catch (SQLException original) { var e = new ServletException("database error"); e.initCause(original); throw e; } When the exception is caught, the original exception can be retrieved: Throwable original = caughtException.getCause(); This wrapping technique is highly recommended. It allows you to throw highlevel exceptions in subsystems without losing the details of the original failure. TIP: The wrapping technique is also useful if a checked exception occurs in a method that is not allowed to throw a checked exception. You can catch the checked exception and wrap it into a runtime exception. Sometimes, you just want to log an exception and rethrow it without any change: try { access the database } From the Library of Hristo Dimov Hristov 386 Chapter 7 Exceptions, Assertions, and Logging catch (Exception e) { logger.log(level, message, e); throw e; } Before Java 7, there was a problem with this approach. Suppose the code is inside a method public void updateRecord() throws SQLException The Java compiler looked at the throw statement inside the catch block, then at the type of e, and complained that this method might throw any Exception, not just a SQLException. This has now been improved. The compiler now tracks the fact that e originates from the try block. Provided that the only checked exceptions in that block are SQLException instances, and provided that e is not changed in the catch block, it is valid to declare the enclosing method as throws SQLException. 7.2.4 The finally Clause When your code throws an exception, it stops processing the remaining code in your method and exits the method. This is a problem if the method has acquired some local resource, which only this method knows about, and that resource must be cleaned up. One solution is to catch all exceptions, carry out the cleanup, and rethrow the exceptions. But this solution is tedious because you need to clean up the resource allocation in two places—in the normal code and in the exception code. The finally clause can solve this problem. NOTE: Since Java 7, there is a more elegant solution, the try-with-resources statement that you will see in the following section. We discuss the finally mechanism in detail because it is the conceptual foundation. But in practice, you will probably use try-with-resources statements more often than finally clauses. The code in the finally clause executes whether or not an exception was caught. In the following example, the program will close the input stream under all circumstances: var in = new FileInputStream(. . .); try { // 1 code that might throw exceptions From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 387 // 2 } catch (IOException e) { // 3 show error message // 4 } finally { // 5 in.close(); } // 6 Let us look at the three possible situations in which the program will execute the finally clause. 1. 2. The code throws no exceptions. In this case, the program first executes all the code in the try block. Then, it executes the code in the finally clause. Afterwards, execution continues with the first statement after the finally clause. In other words, execution passes through points 1, 2, 5, and 6. The code throws an exception that is caught in a catch clause—in our case, an IOException. For this, the program executes all code in the try block, up to the point at which the exception was thrown. The remaining code in the try block is skipped. The program then executes the code in the matching catch clause, and then the code in the finally clause. If the catch clause does not throw an exception, the program executes the first line after the finally clause. In this scenario, execution passes through points 1, 3, 4, 5, and 6. If the catch clause throws an exception, then the exception is thrown back to the caller of this method, and execution passes through points 1, 3, and 5 only. 3. The code throws an exception that is not caught in any catch clause. Here, the program executes all code in the try block until the exception is thrown. The remaining code in the try block is skipped. Then, the code in the finally clause is executed, and the exception is thrown back to the caller of this method. Execution passes through points 1 and 5 only. You can use the finally clause without a catch clause. For example, consider the following try statement: From the Library of Hristo Dimov Hristov 388 Chapter 7 Exceptions, Assertions, and Logging InputStream in = . . .; try { code that might throw exceptions } finally { in.close(); } The in.close() statement in the finally clause is executed whether or not an exception is encountered in the try block. Of course, if an exception is encountered, it is rethrown and must be caught in another catch clause. InputStream in = . . .; try { try { code that might throw exceptions } finally { in.close(); } } catch (IOException e) { show error message } The inner try block has a single responsibility: to make sure that the input stream is closed. The outer try block has a single responsibility: to ensure that errors are reported. Not only is this solution clearer, it is also more functional: Errors in the finally clause are reported. CAUTION: A finally clause can yield unexpected results when it contains return statements. Suppose you exit the middle of a try block with a return statement. Before the method returns, the finally block is executed. If the finally block also contains a return statement, then it masks the original return value. Consider this example: public static int parseInt(String s) { try { return Integer.parseInt(s); } From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 389 finally { return 0; // ERROR } } It looks as if in the call parseInt("42"), the body of the try block returns the integer 42. However, the finally clause is executed before the method actually returns and causes the method to return 0, ignoring the original return value. And it gets worse. Consider the call parseInt("zero"). The Integer.parseInt method throws a NumberFormatException. Then the finally clause is executed, and the return statement swallows the exception! The body of the finally clause is intended for cleaning up resources. Don’t put statements that change the control flow (return, throw, break, continue) inside a finally clause. 7.2.5 The try-with-Resources Statement As of Java 7, there is a useful shortcut to the code pattern open a resource try { work with the resource } finally { close the resource } provided the resource belongs to a class that implements the AutoCloseable interface. That interface has a single method void close() throws Exception NOTE: There is also a Closeable interface. It is a subinterface of AutoCloseable, also with a single close method. However, that method is declared to throw an IOException. In its simplest variant, the try-with-resources statement has the form try (Resource res = . . .) { work with res } From the Library of Hristo Dimov Hristov 390 Chapter 7 Exceptions, Assertions, and Logging When the try block exits, then res.close() is called automatically. Here is a typical example—reading all words of a file: try (var in = new Scanner( new FileInputStream("/usr/share/dict/words"), StandardCharsets.UTF_8)) { while (in.hasNext()) System.out.println(in.next()); } When the block exits normally, or when there was an exception, the in.close() method is called, exactly as if you had used a finally block. You can specify multiple resources. For example, try (var in = new Scanner( new FileInputStream("/usr/share/dict/words"), StandardCharsets.UTF_8); var out = new PrintWriter("out.txt", StandardCharsets.UTF_8)) { while (in.hasNext()) out.println(in.next().toUpperCase()); } No matter how the block exits, both in and out are closed. If you programmed this by hand, you would have needed two nested try/finally statements. As of Java 9, you can provide previously declared effectively final variables in the try header: public static void printAll(String[] lines, PrintWriter out) { try (out) { // effectively final variable for (String line : lines) out.println(line); } // out.close() called here } A difficulty arises when the try block throws an exception and the close method also throws an exception. The try-with-resources statement handles this situation quite elegantly. The original exception is rethrown, and any exceptions thrown by close methods are considered “suppressed.” They are automatically caught and added to the original exception with the addSuppressed method. If you are interested in them, call the getSuppressed method which yields an array of the suppressed expressions from close methods. You don’t want to program this by hand. Use the try-with-resources statement whenever you need to close a resource. From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 391 NOTE: A try-with-resources statement can itself have catch clauses and even a finally clause. These are executed after closing the resources. 7.2.6 Analyzing Stack Trace Elements A stack trace is a listing of all pending method calls at a particular point in the execution of a program. You have almost certainly seen stack trace listings—they are displayed whenever a Java program terminates with an uncaught exception. You can access the text description of a stack trace by calling the printStackTrace method of the Throwable class. var t = new Throwable(); var out = new StringWriter(); t.printStackTrace(new PrintWriter(out)); String description = out.toString(); A more flexible approach is the StackWalker class that yields a stream of StackWalker.StackFrame instances, each describing one stack frame. You can iterate over the stack frames with this call: StackWalker walker = StackWalker.getInstance(); walker.forEach(frame -> analyze frame) If you want to process the Stream<StackWalker.StackFrame> lazily, call walker.walk(stream -> process stream) Stream processing is described in detail in Chapter 1 of Volume II. The StackWalker.StackFrame class has methods to obtain the file name and line number, as well as the class object and method name, of the executing line of code. The toString method yields a formatted string containing all of this information. NOTE: Prior to Java 9, the Throwable.getStackTrace method yielded a StackTraceElement[] array with similar information as the stream of StackWalker.StackFrame instances. However, that call is less efficient since it captures the entire stack even though the caller may only need a few frames, and it only provides access to the class names, but not the class objects, of the pending methods. From the Library of Hristo Dimov Hristov 392 Chapter 7 Exceptions, Assertions, and Logging Listing 7.1 prints the stack trace of a recursive factorial function. For example, if you compute factorial(3), the printout is factorial(3): stackTrace.StackTraceTest.factorial(StackTraceTest.java:20) stackTrace.StackTraceTest.main(StackTraceTest.java:36) factorial(2): stackTrace.StackTraceTest.factorial(StackTraceTest.java:20) stackTrace.StackTraceTest.factorial(StackTraceTest.java:26) stackTrace.StackTraceTest.main(StackTraceTest.java:36) factorial(1): stackTrace.StackTraceTest.factorial(StackTraceTest.java:20) stackTrace.StackTraceTest.factorial(StackTraceTest.java:26) stackTrace.StackTraceTest.factorial(StackTraceTest.java:26) stackTrace.StackTraceTest.main(StackTraceTest.java:36) return 1 return 2 return 6 Listing 7.1 stackTrace/StackTraceTest.java 1 package stackTrace; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 /** * A program that displays a trace feature of a recursive method call. * @version 1.10 2017-12-14 * @author Cay Horstmann */ public class StackTraceTest { /** * Computes the factorial of a number * @param n a non-negative integer * @return n! = 1 * 2 * . . . * n */ public static int factorial(int n) { System.out.println("factorial(" + n + "):"); var walker = StackWalker.getInstance(); walker.forEach(System.out::println); int r; if (n <= 1) r = 1; else r = n * factorial(n - 1); System.out.println("return " + r); return r; } From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 393 28 public static void main(String[] args) { try (var in = new Scanner(System.in)) { System.out.print("Enter n: "); int n = in.nextInt(); factorial(n); } } 29 30 31 32 33 34 35 36 37 38 } java.lang.Throwable 1.0 • Throwable(Throwable cause) 1.4 • Throwable(String message, Throwable cause) 1.4 constructs a Throwable with a given cause. • Throwable initCause(Throwable cause) 1.4 sets the cause for this object or throws an exception if this object already has a cause. Returns this. • Throwable getCause() 1.4 gets the exception object that was set as the cause for this object, or null if no cause was set. • StackTraceElement[] getStackTrace() 1.4 gets the trace of the call stack at the time this object was constructed. • void addSuppressed(Throwable t) 7 adds a “suppressed” exception to this exception. This happens in a try-withresources statement where t is an exception thrown by a close method. • Throwable[] getSuppressed() 7 gets all “suppressed” exceptions of this exception. Typically, these are exceptions thrown by a close method in a try-with-resources statement. java.lang.Exception 1.0 • Exception(Throwable cause) 1.4 • Exception(String message, Throwable cause) constructs an Exception with a given cause. From the Library of Hristo Dimov Hristov 394 Chapter 7 Exceptions, Assertions, and Logging java.lang.RuntimeException 1.0 • RuntimeException(Throwable cause) 1.4 • RuntimeException(String message, Throwable cause) 1.4 constructs a RuntimeException with a given cause. java.lang.StackWalker 9 • static StackWalker getInstance() • static StackWalker getInstance(StackWalker.Option option) • static StackWalker getInstance(Set<StackWalker.Option> options) Gets a StackWalker instance. The options include RETAIN_CLASS_REFERENCE, SHOW_HIDDEN_FRAMES, and SHOW_REFLECT_FRAMES from the StackWalker.Option enumeration. • forEach(Consumer<? super StackWalker.StackFrame> action) carries out the given action on each stack frame, starting with the most recently called method. • walk(Function<? super Stream<StackWalker.StackFrame>,? extends T> function) applies the given function to the stream of stack frames and returns the result of the function. java.lang.StackWalker.StackFrame 9 • String getFileName() gets the name of the source file containing the execution point of this element, or null if the information is not available. • int getLineNumber() gets the line number of the source file containing the execution point of this element, or -1 if the information is not available. • String getClassName() gets the fully qualified name of the class whose method contains the execution point of this element. • String getDeclaringClass() gets the Class object of the method containing the execution point of this element. An exception is thrown if the stack walker was not constructed with the RETAIN_CLASS_REFERENCE option. (Continues) From the Library of Hristo Dimov Hristov 7.2 Catching Exceptions 395 java.lang.StackWalker.StackFrame 9 (Continued) • String getMethodName() gets the name of the method containing the execution point of this element. The name of a constructor is <init>. The name of a static initializer is <clinit>. You can’t distinguish between overloaded methods with the same name. • boolean isNativeMethod() returns true if the execution point of this element is inside a native method. • String toString() returns a formatted string containing the class and method name and the file name and line number, if available. java.lang.StackTraceElement 1.4 • String getFileName() gets the name of the source file containing the execution point of this element, or null if the information is not available. • int getLineNumber() gets the line number of the source file containing the execution point of this element, or -1 if the information is not available. • String getClassName() gets the fully qualified name of the class containing the execution point of this element. • String getMethodName() gets the name of the method containing the execution point of this element. The name of a constructor is <init>. The name of a static initializer is <clinit>. You can’t distinguish between overloaded methods with the same name. • boolean isNativeMethod() returns true if the execution point of this element is inside a native method. • String toString() returns a formatted string containing the class and method name and the file name and line number, if available. From the Library of Hristo Dimov Hristov 396 Chapter 7 Exceptions, Assertions, and Logging 7.3 Tips for Using Exceptions There is a certain amount of controversy about the proper use of exceptions. Some programmers believe that all checked exceptions are a nuisance, others can’t seem to throw enough of them. We think that exceptions (even checked exceptions) have their place, and offer you these tips for their proper use. 1. Exception handling is not supposed to replace a simple test. As an example of this, we wrote some code that tries 10,000,000 times to pop an empty stack. It first does this by finding out whether the stack is empty. if (!s.empty()) s.pop(); Next, we force it to pop the stack no matter what and then catch the EmptyStackException that tells us we should not have done that. try { s.pop(); } catch (EmptyStackException e) { } On our test machine, the version that calls isEmpty ran in 646 milliseconds. The version that catches the EmptyStackException ran in 21,739 milliseconds. As you can see, it took far longer to catch an exception than to perform a simple test. The moral is: Use exceptions for exceptional circumstances only. 2. Do not micromanage exceptions. Many programmers wrap every statement in a separate try block. PrintStream out; Stack s; for (i = 0; i < 100; i++) { try { n = s.pop(); } catch (EmptyStackException e) { // stack was empty } From the Library of Hristo Dimov Hristov 7.3 Tips for Using Exceptions 397 try { out.writeInt(n); } catch (IOException e) { // problem writing to file } } This approach blows up your code dramatically. Think about the task that you want the code to accomplish. Here, we want to pop 100 numbers off a stack and save them to a file. (Never mind why—it is just a toy example.) There is nothing we can do if a problem rears its ugly head. If the stack is empty, it will not become occupied. If the file contains an error, the error will not magically go away. It therefore makes sense to wrap the entire task in a try block. If any one operation fails, you can then abandon the task. try { for (i = 0; i < 100; i++) { n = s.pop(); out.writeInt(n); } } catch { // } catch { // } (IOException e) problem writing to file (EmptyStackException e) stack was empty This code looks much cleaner. It fulfills one of the promises of exception handling: to separate normal processing from error handling. 3. Make good use of the exception hierarchy. Don’t just throw a RuntimeException. Find an appropriate subclass or create your own. Don’t just catch Throwable. It makes your code hard to read and maintain. Respect the difference between checked and unchecked exceptions. Checked exceptions are inherently burdensome—don’t throw them for logic errors. (For example, the reflection library gets this wrong. Callers often need to catch exceptions that they know can never happen.) From the Library of Hristo Dimov Hristov 398 Chapter 7 Exceptions, Assertions, and Logging Do not hesitate to turn an exception into another exception that is more appropriate. For example, when you parse an integer in a file, catch the NumberFormatException and turn it into a subclass of IOException or MySubsystemException. 4. Do not squelch exceptions. In Java, there is a tremendous temptation to shut up exceptions. If you’re writing a method that calls a method that might throw an exception once a century, the compiler whines because you have not declared the exception in the throws list of your method. You do not want to put it in the throws list because then the compiler will whine about all the methods that call your method. So you just shut it up: public Image loadImage(String s) { try { code that threatens to throw checked exceptions } catch (Exception e) {} // so there } Now your code will compile without a hitch. It will run fine, except when an exception occurs. Then, the exception will be silently ignored. If you believe that exceptions are at all important, you should make some effort to handle them right. 5. When you detect an error, “tough love” works better than indulgence. Some programmers worry about throwing exceptions when they detect errors. Maybe it would be better to return a dummy value rather than throw an exception when a method is called with invalid parameters? For example, should Stack.pop return null, or throw an exception when a stack is empty? We think it is better to throw a EmptyStackException at the point of failure than to have a NullPointerException occur at later time. 6. Propagating exceptions is not a sign of shame. Many programmers feel compelled to catch all exceptions that are thrown. If they call a method that throws an exception, such as the FileInputStream constructor or the readLine method, they instinctively catch the exception that may be generated. Often, it is actually better to propagate the exception instead of catching it: public void readStuff(String filename) throws IOException // not a sign of shame! { var in = new FileInputStream(filename, StandardCharsets.UTF_8); From the Library of Hristo Dimov Hristov 7.4 Using Assertions 399 . . . } Higher-level methods are often better equipped to inform the user of errors or to abandon unsuccessful commands. NOTE: Rules 5 and 6 can be summarized as “throw early, catch late.” 7.4 Using Assertions Assertions are a commonly used idiom of defensive programming. In the following sections, you will learn how to use them effectively. 7.4.1 The Assertion Concept Suppose you are convinced that a particular property is fulfilled, and you rely on that property in your code. For example, you may be computing double y = Math.sqrt(x); You are certain that x is not negative. Perhaps it is the result of another computation that can’t have a negative result, or it is a parameter of a method that requires its callers to supply only positive inputs. Still, you want to doublecheck rather than allow confusing “not a number” floating-point values creep into your computation. You could, of course, throw an exception: if (x < 0) throw new IllegalArgumentException("x < 0"); But this code stays in the program, even after testing is complete. If you have lots of checks of this kind, the program may run quite a bit slower than it should. The assertion mechanism allows you to put in checks during testing and to have them automatically removed in the production code. The Java language has a keyword assert. There are two forms: assert condition; and assert condition : expression; Both statements evaluate the condition and throw an AssertionError if it is false. In the second statement, the expression is passed to the constructor of the AssertionError object and turned into a message string. From the Library of Hristo Dimov Hristov 400 Chapter 7 Exceptions, Assertions, and Logging NOTE: The sole purpose of the expression part is to produce a message string. The AssertionError object does not store the actual expression value, so you can’t query it later. As the JDK documentation states, doing so “would encourage programmers to attempt to recover from assertion failure, which defeats the purpose of the facility.” To assert that x is non-negative, you can simply use the statement assert x >= 0; Or you can pass the actual value of x into the AssertionError object, so that it gets displayed later. assert x >= 0 : x; C++ NOTE: The assert macro of the C language turns the assertion condition into a string that is printed if the assertion fails. For example, if assert(x >= 0) fails, it prints that "x >= 0" is the failing condition. In Java, the condition is not automatically part of the error report. If you want to see it, you have to pass it as a string into the AssertionError object: assert x >= 0 : "x >= 0". 7.4.2 Assertion Enabling and Disabling By default, assertions are disabled. Enable them by running the program with the -enableassertions or -ea option: java -enableassertions MyApp Note that you do not have to recompile your program to enable or disable assertions. Enabling or disabling assertions is a function of the class loader. When assertions are disabled, the class loader strips out the assertion code so that it won’t slow execution. You can even turn on assertions in specific classes or in entire packages. For example: java -ea:MyClass -ea:com.mycompany.mylib MyApp This command turns on assertions for the class MyClass and all classes in the package and its subpackages. The option -ea... turns on assertions in all classes of the unnamed package. com.mycompany.mylib You can also disable assertions in certain classes and packages with the -disableassertions or -da option: java -ea:... -da:MyClass MyApp From the Library of Hristo Dimov Hristov 7.4 Using Assertions 401 Some classes are not loaded by a class loader but directly by the virtual machine. You can use these switches to selectively enable or disable assertions in those classes. However, the -ea and -da switches that enable or disable all assertions do not apply to the “system classes” without class loaders. Use the -enablesystemassertions/ -esa switch to enable assertions in system classes. It is also possible to programmatically control the assertion status of class loaders. See the API notes at the end of this section. 7.4.3 Using Assertions for Parameter Checking The Java language gives you three mechanisms to deal with system failures: • Throwing an exception • Logging • Using assertions When should you choose assertions? Keep these points in mind: • Assertion failures are intended to be fatal, unrecoverable errors. • Assertion checks are turned on only during development and testing. (This is sometimes jokingly described as “wearing a life jacket when you are close to shore, and throwing it overboard once you are in the middle of the ocean.”) Therefore, you would not use assertions for signaling recoverable conditions to another part of the program or for communicating problems to the program user. Assertions should only be used to locate internal program errors during testing. Let’s look at a common scenario—the checking of method parameters. Should you use assertions to check for illegal index values or null references? To answer that question, you have to look at the documentation of the method. Suppose you implement a sorting method. /** Sorts the specified range of the specified array in ascending numerical order. The range to be sorted extends from fromIndex, inclusive, to toIndex, exclusive. @param a the array to be sorted. @param fromIndex the index of the first element (inclusive) to be sorted. @param toIndex the index of the last element (exclusive) to be sorted. @throws IllegalArgumentException if fromIndex > toIndex @throws ArrayIndexOutOfBoundsException if fromIndex < 0 or toIndex > a.length */ static void sort(int[] a, int fromIndex, int toIndex) From the Library of Hristo Dimov Hristov 402 Chapter 7 Exceptions, Assertions, and Logging The documentation states that the method throws an exception if the index values are incorrect. That behavior is part of the contract that the method makes with its callers. If you implement the method, you have to respect that contract and throw the indicated exceptions. It would not be appropriate to use assertions instead. Should you assert that a is not null? That is not appropriate either. The method documentation is silent on the behavior of the method when a is null. The callers have the right to assume that the method will return successfully in that case and not throw an assertion error. However, suppose the method contract had been slightly different: @param a the array to be sorted (must not be null). Now the callers of the method have been put on notice that it is illegal to call the method with a null array. Then the method may start with the assertion assert a != null; Computer scientists call this kind of contract a precondition. The original method had no preconditions on its parameters—it promised a well-defined behavior in all cases. The revised method has a single precondition: that a is not null. If the caller fails to fulfill the precondition, then all bets are off and the method can do anything it wants. In fact, with the assertion in place, the method has a rather unpredictable behavior when it is called illegally. It sometimes throws an assertion error, and sometimes a null pointer exception, depending on how its class loader is configured. 7.4.4 Using Assertions for Documenting Assumptions Many programmers use comments to document their underlying assumptions. Consider this example from http://docs.oracle.com/javase/8/docs/technotes/guides /language/assert.html: if (i % 3 == 0) . . . else if (i % 3 == 1) . . . else // (i % 3 == 2) . . . In this case, it makes a lot of sense to use an assertion instead. if (i % 3 == 0) . . . else if (i % 3 == 1) . . . else From the Library of Hristo Dimov Hristov 7.5 Logging 403 { assert i % 3 == 2; . . . } Of course, it would make even more sense to think through the issue thoroughly. What are the possible values of i % 3? If i is positive, the remainders must be 0, 1, or 2. If i is negative, then the remainders can be -1 or -2. Thus, the real assumption is that i is not negative. A better assertion would be assert i >= 0; before the if statement. At any rate, this example shows a good use of assertions as a self-check for the programmer. As you can see, assertions are a tactical tool for testing and debugging. In contrast, logging is a strategic tool for the entire lifecycle of a program. We will examine logging in the next section. java.lang.ClassLoader 1.0 • void setDefaultAssertionStatus(boolean b) 1.4 enables or disables assertions for all classes loaded by this class loader that don’t have an explicit class or package assertion status. • void setClassAssertionStatus(String className, boolean b) 1.4 enables or disables assertions for the given class and its inner classes. • void setPackageAssertionStatus(String packageName, boolean b) 1.4 enables or disables assertions for all classes in the given package and its subpackages. • void clearAssertionStatus() 1.4 removes all explicit class and package assertion status settings and disables assertions for all classes loaded by this class loader. 7.5 Logging Every Java programmer is familiar with the process of inserting calls to System.out.println into troublesome code to gain insight into program behavior. Of course, once you have figured out the cause of trouble, you remove the print statements, only to put them back in when the next problem surfaces. The logging API is designed to overcome this problem. Here are the principal advantages of the API: From the Library of Hristo Dimov Hristov 404 Chapter 7 Exceptions, Assertions, and Logging • It is easy to suppress all log records or just those below a certain level, and just as easy to turn them back on. • Suppressed logs are very cheap, so there is only a minimal penalty for leaving the logging code in your application. • Log records can be directed to different handlers—for displaying in the console, writing to a file, and so on. • Both loggers and handlers can filter records. Filters can discard boring log entries, using any criteria supplied by the filter implementor. • Log records can be formatted in different ways—for example, in plain text or XML. • Applications can use multiple loggers, with hierarchical names such as com.mycompany.myapp, similar to package names. • The logging configuration is controlled by a configuration file. NOTE: Many applications use other logging frameworks, such as Log4J 2 (https://logging.apache.org/log4j/2.x) and Logback (https://logback.qos.ch), that offer higher performance than the standard Java logging framework. These frameworks have slightly different APIs. Logging façades such as SLF4J (https://www.slf4j.org) and Commons Logging (https://commons.apache.org/proper /commons-logging) provide a unified API so that you can replace the logging framework without rewriting your application. To make matters more confusing, Log4J 2 can also be a façade to components that use SLF4J. In this book, we cover the standard Java logging framework. It is good enough for many purposes, and learning its API will prepare you for understanding the alternatives. NOTE: As of Java 9, the Java platform has a separate lightweight logging system that does not depend on the java.logging module (which contains the standard Java logging framework). This system is intended only for use in the Java API. If the java.logging module is present, log messages are automatically forwarded to it. Third-party log frameworks can provide adapters to receive platform logging messages. We do not cover platform logging since it is not meant to be used by application programmers. 7.5.1 Basic Logging For simple logging, use the global logger and call its info method: Logger.getGlobal().info("File->Open menu item selected"); By default, the record is printed like this: From the Library of Hristo Dimov Hristov 7.5 Logging 405 May 10, 2013 10:12:15 PM LoggingImageViewer fileOpen INFO: File->Open menu item selected But if you call Logger.getGlobal().setLevel(Level.OFF); at an appropriate place (such as the beginning of main), all logging is suppressed. 7.5.2 Advanced Logging Now that you have seen “logging for dummies,” let’s go on to industrialstrength logging. In a professional application, you wouldn’t want to log all records to a single global logger. Instead, you can define your own loggers. Call the getLogger method to create or retrieve a logger: private static final Logger myLogger = Logger.getLogger("com.mycompany.myapp"); TIP: A logger that is not referenced by any variable can be garbage-collected. To prevent this, save a reference to the logger with a static variable, as in the example above. Similar to package names, logger names are hierarchical. In fact, they are more hierarchical than packages. There is no semantic relationship between a package and its parent, but logger parents and children share certain properties. For example, if you set the log level on the logger "com.mycompany", then the child loggers inherit that level. There are seven logging levels: • • • • • • • SEVERE WARNING INFO CONFIG FINE FINER FINEST By default, the top three levels are actually logged. You can set a different level—for example, logger.setLevel(Level.FINE); From the Library of Hristo Dimov Hristov 406 Chapter 7 Exceptions, Assertions, and Logging Now FINE and all levels above it are logged. You can also use Level.ALL to turn on logging for all levels or Level.OFF to turn all logging off. There are logging methods for all levels, such as logger.warning(message); logger.fine(message); and so on. Alternatively, you can use the log method and supply the level, such as logger.log(Level.FINE, message); TIP: The default logging configuration logs all records with the level of INFO or higher. Therefore, you should use the levels CONFIG, FINE, FINER, and FINEST for debugging messages that are useful for diagnostics but meaningless to the user. CAUTION: If you set the logging level to a value finer than INFO, you also need to change the log handler configuration. The default log handler suppresses messages below INFO. See the next section for details. The default log record shows the name of the class and method that contain the logging call, as inferred from the call stack. However, if the virtual machine optimizes execution, accurate call information may not be available. You can use the logp method to give the precise location of the calling class and method. The method signature is void logp(Level l, String className, String methodName, String message) There are convenience methods for tracing execution flow: void void void void void entering(String className, String methodName) entering(String className, String methodName, Object param) entering(String className, String methodName, Object[] params) exiting(String className, String methodName) exiting(String className, String methodName, Object result) For example: int read(String file, String pattern) { logger.entering("com.mycompany.mylib.Reader", "read", new Object[] { file, pattern }); . . . logger.exiting("com.mycompany.mylib.Reader", "read", count); From the Library of Hristo Dimov Hristov 7.5 Logging 407 return count; } These calls generate log records of level FINER that start with the strings ENTRY and RETURN. NOTE: At some point in the future, the logging methods with an Object[] parameter will be rewritten to support variable parameter lists (“varargs”). Then, you will be able to make calls such as logger.entering("com.mycompany.mylib.Reader", "read", file, pattern). A common use for logging is to log unexpected exceptions. Two convenience methods include a description of the exception in the log record. void throwing(String className, String methodName, Throwable t) void log(Level l, String message, Throwable t) Typical uses are if (. . .) { var e = new IOException(". . ."); logger.throwing("com.mycompany.mylib.Reader", "read", e); throw e; } and try { . . . } catch (IOException e) { Logger.getLogger("com.mycompany.myapp").log(Level.WARNING, "Reading image", e); } The throwing call logs a record with level FINER and a message that starts with THROW. 7.5.3 Changing the Log Manager Configuration You can change various properties of the logging system by editing a configuration file. The default configuration file is located at jdk/conf /logging.properties (or at jre/lib/logging.properties prior to Java 9). From the Library of Hristo Dimov Hristov 408 Chapter 7 Exceptions, Assertions, and Logging To use another file, set the java.util.logging.config.file property to the file location by starting your application with java -Djava.util.logging.config.file=configFile MainClass To change the default logging level, edit the configuration file and modify the line .level=INFO You can specify the logging levels for your own loggers by adding lines such as com.mycompany.myapp.level=FINE That is, append the .level suffix to the logger name. As you will see later in this section, the loggers don’t actually send the messages to the console—that is the job of the handlers. Handlers also have levels. To see FINE messages on the console, you also need to set java.util.logging.ConsoleHandler.level=FINE CAUTION: The settings in the log manager configuration are not system properties. Starting a program with -Dcom.mycompany.myapp.level=FINE does not have any effect on the logger. The log manager is initialized during VM startup, before main executes. If you want to customize the logging properties but didn’t start your application with the -Djava.util.logging.config.file command-line option, call System.setProperty( "java.util.logging.config.file", file) in your program. But then you must also call LogManager.getLogManager().readConfiguration() to reinitialize the log manager. As of Java 9, you can instead update the logging configuration by calling LogManager.getLogManager().updateConfiguration(mapper); A new configuration is read from the location specified by the java.util.logging.config.file system property. Then the mapper is applied to resolve the values for all keys in the old or new configuration. The mapper is a Function<String,BiFunction<String,String,String>>. It maps keys in the existing configuration to replacement functions. Each replacement function receives the old and new values associated with the key (or null if there is no associated value), and produces a replacement, or null if the key should be dropped in the update. That sounds rather complex, so let’s walk through a couple of examples. A useful mapping scheme would be to merge the old and new configurations, From the Library of Hristo Dimov Hristov 7.5 Logging 409 preferring the new value when a key is present in both the old and new configurations. Then the mapper is key -> ((oldValue, newValue) -> newValue == null ? oldValue : newValue) Or perhaps you want to only update the keys that start with com.mycompany and leave the others unchanged: key -> key.startsWith("com.mycompany") ? ((oldValue, newValue) -> newValue) : ((oldValue, newValue) -> oldValue) It is also possible to change logging levels in a running program by using the jconsole program. See www.oracle.com/technetwork/articles/java/jconsole-1564139.html #LoggingControl for information. NOTE: The logging properties file is processed by the java.util.logging.LogManager class. It is possible to specify a different log manager by setting the java.util .logging.manager system property to the name of a subclass. Alternatively, you can keep the standard log manager and still bypass the initialization from the logging properties file. Set the java.util.logging.config.class system property to the name of a class that sets log manager properties in some other way. See the API documentation for the LogManager class for more information. 7.5.4 Localization You may want to localize logging messages so that they are readable for international users. Internationalization of applications is the topic of Chapter 7 of Volume II. Briefly, here are the points to keep in mind when localizing logging messages. Localized applications contain locale-specific information in resource bundles. A resource bundle consists of a set of mappings for various locales (such as United States or Germany). For example, a resource bundle may map the string "readingFile" into strings "Reading file" in English or "Achtung! Datei wird eingelesen" in German. A program may contain multiple resource bundles—for example, one for menus and another for log messages. Each resource bundle has a name (such as "com.mycompany.logmessages"). To add mappings to a resource bundle, supply a file for each locale. English message mappings are in a file com/mycompany/logmessages_en.properties, and German message mappings are in a file com/mycompany/logmessages_de.properties. (The en and de are the language codes.) You place the files together with the class files of your application, so that the From the Library of Hristo Dimov Hristov 410 Chapter 7 Exceptions, Assertions, and Logging class will automatically locate them. These files are plain text files, consisting of entries such as ResourceBundle readingFile=Achtung! Datei wird eingelesen renamingFile=Datei wird umbenannt . . . When requesting a logger, you can specify a resource bundle: Logger logger = Logger.getLogger(loggerName, "com.mycompany.logmessages"); Then specify the resource bundle key, not the actual message string, for the log message: logger.info("readingFile"); You often need to include arguments into localized messages. A message may contain placeholders: {0}, {1}, and so on. For example, to include the file name with a log message, use the placeholder like this: Reading file {0}. Achtung! Datei {0} wird eingelesen. Then, to pass values into the placeholders, call one of the following methods: logger.log(Level.INFO, "readingFile", fileName); logger.log(Level.INFO, "renamingFile", new Object[] { oldName, newName }); Alternatively, as of Java 9, you can specify the resource bundle object (and not the name) in the logrb method: logger.logrb(Level.INFO, bundle, "renamingFile", oldName, newName); NOTE: This is the only logging method that uses variable arguments for the message parameters. 7.5.5 Handlers By default, loggers send records to a ConsoleHandler that prints them to the stream. Specifically, the logger sends the record to the parent handler, and the ultimate ancestor (with name "") has a ConsoleHandler. System.err Like loggers, handlers have a logging level. For a record to be logged, its logging level must be above the threshold of both the logger and the handler. The log manager configuration file sets the logging level of the default console handler as java.util.logging.ConsoleHandler.level=INFO From the Library of Hristo Dimov Hristov 7.5 Logging 411 To log records with level FINE, change both the default logger level and the handler level in the configuration. Alternatively, you can bypass the configuration file altogether and install your own handler. Logger logger = Logger.getLogger("com.mycompany.myapp"); logger.setLevel(Level.FINE); logger.setUseParentHandlers(false); var handler = new ConsoleHandler(); handler.setLevel(Level.FINE); logger.addHandler(handler); By default, a logger sends records both to its own handlers and to the handlers of the parent. Our logger is a child of the primordial logger (with name "") that sends all records with level INFO and above to the console. We don’t want to see those records twice, however, so we set the useParentHandlers property to false. To send log records elsewhere, add another handler. The logging API provides two useful handlers for this purpose: a FileHandler and a SocketHandler. The SocketHandler sends records to a specified host and port. Of greater interest is the FileHandler that collects records in a file. You can simply send records to a default file handler, like this: var handler = new FileHandler(); logger.addHandler(handler); The records are sent to a file javan.log in the user’s home directory, where n is a number to make the file unique. If a system has no concept of the user’s home directory (for example, in Windows 95/98/ME), then the file is stored in a default location such as C:\Windows. By default, the records are formatted in XML. A typical log record has the form <record> <date>2002-02-04T07:45:15</date> <millis>1012837515710</millis> <sequence>1</sequence> <logger>com.mycompany.myapp</logger> <level>INFO</level> <class>com.mycompany.mylib.Reader</class> <method>read</method> <thread>10</thread> <message>Reading file corejava.gif</message> </record> You can modify the default behavior of the file handler by setting various parameters in the log manager configuration (see Table 7.1) or by using another constructor (see the API notes at the end of this section). From the Library of Hristo Dimov Hristov 412 Chapter 7 Exceptions, Assertions, and Logging Table 7.1 File Handler Configuration Parameters Configuration Property Description Default java.util.logging.FileHandler.level The handler level Level.ALL java.util.logging.FileHandler.append Controls whether the handler should append to an existing file, or open a new file for each program run false java.util.logging.FileHandler.limit The approximate maximum number of bytes to write to a file before opening another (0 = no limit) 0 (no limit) in the FileHandler class, 50000 in java.util.logging.FileHandler.pattern The pattern for the log file name. See Table 7.2 for pattern variables. %h/java%u.log java.util.logging.FileHandler.count The number of logs in a rotation sequence 1 (no rotation) java.util.logging.FileHandler.filter The filter class to use No filtering java.util.logging.FileHandler.encoding The character encoding to use The platform encoding java.util.logging.FileHandler.formatter The record formatter java.util.logging.XMLFormatter the default log manager configuration You probably don’t want to use the default log file name. Therefore, you should use another pattern, such as %h/myapp.log. (See Table 7.2 for an explanation of the pattern variables.) If multiple applications (or multiple copies of the same application) use the same log file, you should turn the append flag on. Alternatively, use %u in the file name pattern so that each application creates a unique copy of the log. From the Library of Hristo Dimov Hristov 7.5 Logging 413 Table 7.2 Log File Pattern Variables Variable Description %h The value of the user.home system property %t The system temporary directory %u A unique number to resolve conflicts %g The generation number for rotated logs (a .%g suffix is used if rotation is specified and the pattern doesn’t contain %g) %% The % character It is also a good idea to turn file rotation on. Log files are kept in a rotation sequence, such as myapp.log.0, myapp.log.1, myapp.log.2, and so on. Whenever a file exceeds the size limit, the oldest log is deleted, the other files are renamed, and a new file with generation number 0 is created. TIP: Many programmers use logging as an aid for the technical support staff. If a program misbehaves in the field, the user can send back the log files for inspection. In that case, you should turn the append flag on, use rotating logs, or both. You can also define your own handlers by extending the Handler or the StreamHandler class. We define such a handler in the example program at the end of this section. That handler displays the records in a window (see Figure 7.2). Figure 7.2 A log handler that displays records in a window From the Library of Hristo Dimov Hristov 414 Chapter 7 Exceptions, Assertions, and Logging The handler extends the StreamHandler class and installs a stream whose write methods display the stream output in a text area. class WindowHandler extends StreamHandler { public WindowHandler() { . . . var output = new JTextArea(); setOutputStream(new OutputStream() { public void write(int b) {} // not called public void write(byte[] b, int off, int len) { output.append(new String(b, off, len)); } }); } . . . } There is just one problem with this approach—the handler buffers the records and only writes them to the stream when the buffer is full. Therefore, we override the publish method to flush the buffer after each record: class WindowHandler extends StreamHandler { . . . public void publish(LogRecord record) { super.publish(record); flush(); } } If you want to write more exotic stream handlers, extend the Handler class and define the publish, flush, and close methods. 7.5.6 Filters By default, records are filtered according to their logging levels. Each logger and handler can have an optional filter to perform additional filtering. To define a filter, implement the Filter interface and define the method boolean isLoggable(LogRecord record) From the Library of Hristo Dimov Hristov 7.5 Logging 415 Analyze the log record, using any criteria that you desire, and return true for those records that should be included in the log. For example, a particular filter may only be interested in the messages generated by the entering and exiting methods. The filter should then call record.getMessage() and check whether it starts with ENTRY or RETURN. To install a filter into a logger or handler, simply call the setFilter method. Note that you can have at most one filter at a time. 7.5.7 Formatters The ConsoleHandler and FileHandler classes emit the log records in text and XML formats. However, you can define your own formats as well. You need to extend the Formatter class and override the method String format(LogRecord record) Format the information in the record in any way you like and return the resulting string. In your format method, you may want to call the method String formatMessage(LogRecord record) That method formats the message part of the record, substituting parameters and applying localization. Many file formats (such as XML) require a head and tail parts that surround the formatted records. To achieve this, override the methods String getHead(Handler h) String getTail(Handler h) Finally, call the setFormatter method to install the formatter into the handler. 7.5.8 A Logging Recipe With so many options for logging, it is easy to lose track of the fundamentals. The following recipe summarizes the most common operations. 1. For a simple application, choose a single logger. It is a good idea to give the logger the same name as your main application package, such as com.mycompany.myprog. You can always get the logger by calling Logger logger = Logger.getLogger("com.mycompany.myprog"); For convenience, you may want to add static fields private static final Logger logger = Logger.getLogger("com.mycompany.myprog"); to classes with a lot of logging activity. From the Library of Hristo Dimov Hristov 416 Chapter 7 2. Exceptions, Assertions, and Logging The default logging configuration logs all messages of level INFO or higher to the console. Users can override the default configuration, but as you have seen, the process is a bit involved. Therefore, it is a good idea to install a more reasonable default in your application. The following code ensures that all messages are logged to an applicationspecific file. Place the code into the main method of your application. if (System.getProperty("java.util.logging.config.class") == null && System.getProperty("java.util.logging.config.file") == null) { try { Logger.getLogger("").setLevel(Level.ALL); final int LOG_ROTATION_COUNT = 10; var handler = new FileHandler("%h/myapp.log", 0, LOG_ROTATION_COUNT); Logger.getLogger("").addHandler(handler); } catch (IOException e) { logger.log(Level.SEVERE, "Can't create log file handler", e); } } 3. Now you are ready to log to your heart’s content. Keep in mind that all messages with level INFO, WARNING, and SEVERE show up on the console. Therefore, reserve these levels for messages that are meaningful to the users of your program. The level FINE is a good choice for logging messages that are intended for programmers. Whenever you are tempted to call System.out.println, emit a log message instead: logger.fine("File open dialog canceled"); It is also a good idea to log unexpected exceptions. For example: try { . . . } catch (SomeException e) { logger.log(Level.FINE, "explanation", e); } Listing 7.2 puts this recipe to use with an added twist: Logging messages are also displayed in a log window. From the Library of Hristo Dimov Hristov 7.5 Logging 417 Listing 7.2 logging/LoggingImageViewer.java 1 package logging; 2 3 4 5 6 7 import import import import import java.awt.*; java.awt.event.*; java.io.*; java.util.logging.*; javax.swing.*; 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 /** * A modification of the image viewer program that logs various events. * @version 1.03 2015-08-20 * @author Cay Horstmann */ public class LoggingImageViewer { public static void main(String[] args) { if (System.getProperty("java.util.logging.config.class") == null && System.getProperty("java.util.logging.config.file") == null) { try { Logger.getLogger("com.horstmann.corejava").setLevel(Level.ALL); final int LOG_ROTATION_COUNT = 10; var handler = new FileHandler("%h/LoggingImageViewer.log", 0, LOG_ROTATION_COUNT); Logger.getLogger("com.horstmann.corejava").addHandler(handler); } catch (IOException e) { Logger.getLogger("com.horstmann.corejava").log(Level.SEVERE, "Can't create log file handler", e); } } 34 35 36 37 38 39 EventQueue.invokeLater(() -> { var windowHandler = new WindowHandler(); windowHandler.setLevel(Level.ALL); Logger.getLogger("com.horstmann.corejava").addHandler(windowHandler); 40 41 42 43 var frame = new ImageViewerFrame(); frame.setTitle("LoggingImageViewer"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 44 (Continues) From the Library of Hristo Dimov Hristov 418 Chapter 7 Exceptions, Assertions, and Logging Listing 7.2 (Continued) Logger.getLogger("com.horstmann.corejava").fine("Showing frame"); frame.setVisible(true); 45 46 }); 47 } 48 49 } 50 51 52 53 54 55 56 57 /** * The frame that shows the image. */ class ImageViewerFrame extends JFrame { private static final int DEFAULT_WIDTH = 300; private static final int DEFAULT_HEIGHT = 400; 58 59 60 private JLabel label; private static Logger logger = Logger.getLogger("com.horstmann.corejava"); 61 62 63 64 65 public ImageViewerFrame() { logger.entering("ImageViewerFrame", "<init>"); setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT); 66 67 68 69 // set up menu bar var menuBar = new JMenuBar(); setJMenuBar(menuBar); 70 71 72 var menu = new JMenu("File"); menuBar.add(menu); 73 74 75 76 var openItem = new JMenuItem("Open"); menu.add(openItem); openItem.addActionListener(new FileOpenListener()); 77 78 79 80 81 82 83 84 85 86 87 var exitItem = new JMenuItem("Exit"); menu.add(exitItem); exitItem.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { logger.fine("Exiting."); System.exit(0); } }); 88 89 90 // use a label to display the images label = new JLabel(); From the Library of Hristo Dimov Hristov 7.5 Logging 419 add(label); logger.exiting("ImageViewerFrame", "<init>"); 91 92 } 93 94 private class FileOpenListener implements ActionListener { public void actionPerformed(ActionEvent event) { logger.entering("ImageViewerFrame.FileOpenListener", "actionPerformed", event); 95 96 97 98 99 100 // set up file chooser var chooser = new JFileChooser(); chooser.setCurrentDirectory(new File(".")); 101 102 103 104 // accept all files ending with .gif chooser.setFileFilter(new javax.swing.filechooser.FileFilter() { public boolean accept(File f) { return f.getName().toLowerCase().endsWith(".gif") || f.isDirectory(); } 105 106 107 108 109 110 111 112 public String getDescription() { return "GIF Images"; } 113 114 115 116 }); 117 118 // show file chooser dialog int r = chooser.showOpenDialog(ImageViewerFrame.this); 119 120 121 // if image file accepted, set it as icon of the label if (r == JFileChooser.APPROVE_OPTION) { String name = chooser.getSelectedFile().getPath(); logger.log(Level.FINE, "Reading file {0}", name); label.setIcon(new ImageIcon(name)); } else logger.fine("File open dialog canceled."); logger.exiting("ImageViewerFrame.FileOpenListener", "actionPerformed"); 122 123 124 125 126 127 128 129 130 } 131 } 132 133 } 134 135 136 137 /** * A handler for displaying log records in a window. */ (Continues) From the Library of Hristo Dimov Hristov 420 Chapter 7 Exceptions, Assertions, and Logging Listing 7.2 (Continued) 138 139 140 class WindowHandler extends StreamHandler { private JFrame frame; 141 public WindowHandler() { frame = new JFrame(); var output = new JTextArea(); output.setEditable(false); frame.setSize(200, 200); frame.add(new JScrollPane(output)); frame.setFocusableWindowState(false); frame.setVisible(true); setOutputStream(new OutputStream() { public void write(int b) { } // not called 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 public void write(byte[] b, int off, int len) { output.append(new String(b, off, len)); } 157 158 159 160 }); 161 } 162 163 public void publish(LogRecord record) { if (!frame.isVisible()) return; super.publish(record); flush(); } 164 165 166 167 168 169 170 } java.util.logging.Logger 1.4 • Logger getLogger(String loggerName) • Logger getLogger(String loggerName, String bundleName) gets the logger with the given name. If the logger doesn’t exist, it is created. Localized messages are located in the resource bundle whose name is bundleName. (Continues) From the Library of Hristo Dimov Hristov 7.5 Logging 421 java.util.logging.Logger 1.4 (Continued) • • • • • • • void void void void void void void severe(String message) warning(String message) info(String message) config(String message) fine(String message) finer(String message) finest(String message) logs a record with the level indicated by the method name and the given message. • • • • • void void void void void entering(String className, String methodName) entering(String className, String methodName, Object param) entering(String className, String methodName, Object[] param) exiting(String className, String methodName) exiting(String className, String methodName, Object result) logs a record that describes entering or exiting a method with the given parameter(s) or return value. • void throwing(String className, String methodName, Throwable t) logs a record that describes throwing of the given exception object. • • • • void void void void log(Level log(Level log(Level log(Level level, level, level, level, String String String String message) message, Object obj) message, Object[] objs) message, Throwable t) logs a record with the given level and message, optionally including objects or a throwable. To include objects, the message must contain formatting placeholders ({0}, {1}, and so on). • • • • void void void void logp(Level logp(Level logp(Level logp(Level level, level, level, level, String String String String className, className, className, className, String String String String methodName, methodName, methodName, methodName, String String String String message) message, Object obj) message, Object[] objs) message, Throwable t) logs a record with the given level, precise caller information, and message, optionally including objects or a throwable. (Continues) From the Library of Hristo Dimov Hristov 422 Chapter 7 Exceptions, Assertions, and Logging java.util.logging.Logger 1.4 (Continued) • void logrb(Level level, String className, String methodName, ResourceBundle bundle, String message, Object... params) 9 • void logrb(Level level, String className, String methodName, ResourceBundle bundle, String message, Throwable thrown) 9 logs a record with the given level, precise caller information, resource bundle, and message, followed by objects or a throwable. • Level getLevel() • void setLevel(Level l) gets and sets the level of this logger. • Logger getParent() • void setParent(Logger l) gets and sets the parent logger of this logger. • Handler[] getHandlers() gets all handlers of this logger. • void addHandler(Handler h) • void removeHandler(Handler h) adds or removes a handler for this logger. • boolean getUseParentHandlers() • void setUseParentHandlers(boolean b) gets and sets the “use parent handler” property. If this property is true, the logger forwards all logged records to the handlers of its parent. • Filter getFilter() • void setFilter(Filter f) gets and sets the filter of this logger. java.util.logging.Handler 1.4 • abstract void publish(LogRecord record) sends the record to the intended destination. • abstract void flush() flushes any buffered data. • abstract void close() flushes any buffered data and releases all associated resources. (Continues) From the Library of Hristo Dimov Hristov 7.5 Logging 423 java.util.logging.Handler 1.4 (Continued) • Filter getFilter() • void setFilter(Filter f) gets and sets the filter of this handler. • Formatter getFormatter() • void setFormatter(Formatter f) gets and sets the formatter of this handler. • Level getLevel() • void setLevel(Level l) gets and sets the level of this handler. java.util.logging.ConsoleHandler 1.4 • ConsoleHandler() constructs a new console handler. java.util.logging.FileHandler 1.4 • • • • • FileHandler(String FileHandler(String FileHandler(String FileHandler(String FileHandler(String pattern) pattern, pattern, pattern, pattern, boolean append) int limit, int count) int limit, int count, boolean append) long limit, int count, boolean append) 9 constructs a file handler. See Table 7.2 for the pattern format. limit is the approximate maximum number of bytes before a new log file is opened. count is the number of files in a rotation sequence. If append is true, records should be appended to an existing log file. java.util.logging.LogRecord 1.4 • Level getLevel() gets the logging level of this record. • String getLoggerName() gets the name of the logger that is logging this record. (Continues) From the Library of Hristo Dimov Hristov 424 Chapter 7 Exceptions, Assertions, and Logging java.util.logging.LogRecord 1.4 (Continued) • ResourceBundle getResourceBundle() • String getResourceBundleName() gets the resource bundle, or its name, to be used for localizing the message, or null if none is provided. • String getMessage() gets the “raw” message before localization or formatting. • Object[] getParameters() gets the parameter objects, or null if none is provided. • Throwable getThrown() gets the thrown object, or null if none is provided. • String getSourceClassName() • String getSourceMethodName() gets the location of the code that logged this record. This information may be supplied by the logging code or automatically inferred from the runtime stack. It might be inaccurate if the logging code supplied the wrong value or if the running code was optimized so that the exact location cannot be inferred. • long getMillis() gets the creation time, in milliseconds since 1970. • Instant getInstant() 9 gets the creation time as a java.time.Instant (see Chapter 6 of Volume II). • long getSequenceNumber() gets the unique sequence number of this record. • int getThreadID() gets the unique ID for the thread in which this record was created. These IDs are assigned by the LogRecord class and have no relationship to other thread IDs. java.util.logging.LogManager 1.4 • static LogManager getLogManager() gets the global LogManager instance. (Continues) From the Library of Hristo Dimov Hristov 7.6 Debugging Tips 425 java.util.logging.LogManager 1.4 (Continued) • void readConfiguration() • void readConfiguration(InputStream in) reads the logging configuration from the file specified by the system property java.util.logging.config.file, or the given input stream. • void updateConfiguration(InputStream in, Function<String,BiFunction<String,String,String>> mapper) 9 • void updateConfiguration(Function<String,BiFunction<String,String,String>> mapper) 9 merges the logging configuration with the file specified by the system property java.util.logging.config.file or the given input stream. See Section 7.5.3, “Changing the Log Manager Configuration,” on p. 407 for a description of the mapper parameter. java.util.logging.Filter 1.4 • boolean isLoggable(LogRecord record) returns true if the given log record should be logged. java.util.logging.Formatter 1.4 • abstract String format(LogRecord record) returns the string that results from formatting the given log record. • String getHead(Handler h) • String getTail(Handler h) returns the strings that should appear at the head and tail of the document containing the log records. The Formatter superclass defines these methods to return the empty string; override them if necessary. • String formatMessage(LogRecord record) returns the localized and formatted message part of the log record. 7.6 Debugging Tips Suppose you wrote your program and made it bulletproof by catching and properly handling all exceptions. Then you run it, and it does not work right. Now what? (If you never have this problem, you can skip the remainder of this chapter.) From the Library of Hristo Dimov Hristov 426 Chapter 7 Exceptions, Assertions, and Logging Of course, it is best if you have a convenient and powerful debugger. Debuggers are available as a part of professional development environments such as Eclipse, IntelliJ, and NetBeans. In this section, we offer you a number of tips that may be worth trying before you launch the debugger. 1. You can print or log the value of any variable with code like this: System.out.println("x=" + x); or Logger.getGlobal().info("x=" + x); If x is a number, it is converted to its string equivalent. If x is an object, Java calls its toString method. To get the state of the implicit parameter object, print the state of the this object. Logger.getGlobal().info("this=" + this); Most of the classes in the Java library are very conscientious about overriding the toString method to give you useful information about the class. This is a real boon for debugging. You should make the same effort in your classes. 2. One seemingly little-known but very useful trick is putting a separate main method in each class. Inside it, you can put a unit test stub that lets you test the class in isolation. public class MyClass { methods and fields . . . public static void main(String[] args) { test code } } Make a few objects, call all methods, and check that each of them does the right thing. You can leave all these main methods in place and launch the Java virtual machine separately on each of the files to run the tests. When you run an applet, none of these main methods are ever called. When you run an application, the Java virtual machine calls only the main method of the startup class. 3. If you liked the preceding tip, you should check out JUnit from http://junit.org. JUnit is a very popular unit testing framework that makes it easy to organize suites of test cases. Run the tests whenever you make changes to a class, and add another test case whenever you find a bug. From the Library of Hristo Dimov Hristov 7.6 4. Debugging Tips 427 A logging proxy is an object of a subclass that intercepts method calls, logs them, and then calls the superclass. For example, if you have trouble with the nextDouble method of the Random class, you can create a proxy object as an instance of an anonymous subclass: var generator = new Random() { public double nextDouble() { double result = super.nextDouble(); Logger.getGlobal().info("nextDouble: " + result); return result; } }; Whenever the nextDouble method is called, a log message is generated. To find out who called the method, generate a stack trace. 5. You can get a stack trace from any exception object with the printStackTrace method in the Throwable class. The following code catches any exception, prints the exception object and the stack trace, and rethrows the exception so it can find its intended handler. try { . . . } catch (Throwable t) { t.printStackTrace(); throw t; } You don’t even need to catch an exception to generate a stack trace. Simply insert the statement Thread.dumpStack(); anywhere into your code to get a stack trace. 6. Normally, the stack trace is displayed on System.err. If you want to log or display the stack trace, here is how you can capture it into a string: var out = new StringWriter(); new Throwable().printStackTrace(new PrintWriter(out)); String description = out.toString(); 7. It is often handy to trap program errors in a file. However, errors are sent to System.err, not System.out. Therefore, you cannot simply trap them by running From the Library of Hristo Dimov Hristov 428 Chapter 7 Exceptions, Assertions, and Logging java MyProgram > errors.txt Instead, capture the error stream as java MyProgram 2> errors.txt To capture both System.err and System.out in the same file, use java MyProgram 1> errors.txt 2>&1 This works in bash and the Windows shell. 8. Having the stack traces of uncaught exceptions show up in System.err is not ideal. These messages are confusing to end users if they happen to see them, and they are not available for diagnostic purposes when you need them. A better approach is to log them to a file. You can change the handler for uncaught exceptions with the static Thread.setDefaultUncaughtExceptionHandler method: Thread.setDefaultUncaughtExceptionHandler( new Thread.UncaughtExceptionHandler() { public void uncaughtException(Thread t, Throwable e) { save information in log file }; }); 9. To watch class loading, launch the Java virtual machine with the -verbose flag. You will get a printout such as the following: [0.012s][info][class,load] [0.034s][info][class,load] [0.035s][info][class,load] [0.035s][info][class,load] [0.035s][info][class,load] [0.035s][info][class,load] [0.036s][info][class,load] [0.036s][info][class,load] [0.036s][info][class,load] [0.036s][info][class,load] [0.036s][info][class,load] [0.037s][info][class,load] [0.037s][info][class,load] [0.037s][info][class,load] [0.037s][info][class,load] [0.037s][info][class,load] [0.037s][info][class,load] [0.037s][info][class,load] [0.038s][info][class,load] . . . opened: /opt/jdk-11.0.1/lib/modules java.lang.Object source: jrt:/java.base java.io.Serializable source: jrt:/java.base java.lang.Comparable source: jrt:/java.base java.lang.CharSequence source: jrt:/java.base java.lang.String source: jrt:/java.base java.lang.reflect.AnnotatedElement source: jrt:/java.base java.lang.reflect.GenericDeclaration source: jrt:/java.base java.lang.reflect.Type source: jrt:/java.base java.lang.Class source: jrt:/java.base java.lang.Cloneable source: jrt:/java.base java.lang.ClassLoader source: jrt:/java.base java.lang.System source: jrt:/java.base java.lang.Throwable source: jrt:/java.base java.lang.Error source: jrt:/java.base java.lang.ThreadDeath source: jrt:/java.base java.lang.Exception source: jrt:/java.base java.lang.RuntimeException source: jrt:/java.base java.lang.SecurityManager source: jrt:/java.base From the Library of Hristo Dimov Hristov 7.6 Debugging Tips 429 This can occasionally be helpful to diagnose class path problems. 10. The -Xlint option tells the compiler to spot common code problems. For example, if you compile with the command javac -Xlint sourceFiles the compiler will report missing break statements in switch statements. (The term “lint” originally described a tool for locating potential problems in C programs, but is now generically applied to any tools that flag constructs that are questionable but not illegal.) You will get messages such as warning: [fallthrough] possible fall-through into case The string in square brackets identifies the warning category. You can enable and disable each category. Since most of them are quite useful, it seems best to leave them all in place and disable only those that you don’t care about, like this: javac -Xlint:all,-fallthrough,-serial sourceFiles You get a list of all warnings from the command javac --help -X 11. The Java VM has support for monitoring and management of Java applications, allowing the installation of agents in the virtual machine that track memory consumption, thread usage, class loading, and so on. This feature is particularly important for large and long-running Java programs, such as application servers. As a demonstration of these capabilities, the JDK ships with a graphical tool called jconsole that displays statistics about the performance of a virtual machine (see Figure 7.3). Start your program, then start jconsole and pick your program from the list of running Java programs. The console gives you a wealth of information about your running program. See www.oracle.com/technetwork/articles/java/jconsole-1564139.html for more information. 12. Java Mission Control is a professional-level profiling and diagnostics tool that is included with the Oracle JDK and is free to use for development purposes. A commercial license is required for use in production. An open source version will be a part of the OpenJDK at some point. Like jconsole, Java Mission Control can attach to a running virtual machine. It can also analyze the output from Java Flight Recorder, a tool that collects diagnostic and profiling data from a running Java application. From the Library of Hristo Dimov Hristov 430 Chapter 7 Exceptions, Assertions, and Logging See https://docs.oracle.com/javacomponents/index.html for more information about these tools. Figure 7.3 The jconsole program This chapter introduced you to exception handling and logging. You also saw useful hints for testing and debugging. The next two chapters cover generic programming and its most important application: the Java collections framework. From the Library of Hristo Dimov Hristov CHAPTER 8 Generic Programming In this chapter • 8.1 Why Generic Programming?, page 432 • 8.2 Defining a Simple Generic Class, page 434 • 8.3 Generic Methods, page 437 • 8.4 Bounds for Type Variables, page 438 • 8.5 Generic Code and the Virtual Machine, page 441 • 8.6 Restrictions and Limitations, page 447 • 8.7 Inheritance Rules for Generic Types, page 457 • 8.8 Wildcard Types, page 459 • 8.9 Reflection and Generics, page 467 Generic classes and methods have type parameters. This allows them to describe precisely what should happen when they are instantiated with specific types. Prior to generic classes, programmers had to use the Object for writing code that works with multiple types. This was both cumbersome and unsafe. With the introduction of generics, Java has an expressive type system that allows designers to describe in detail how types of variables and methods should vary. In straightforward situations, you will find it simple to implement generic code. In more advanced cases, it can get quite complex—for implemetors. The goal is to provide classes and methods that other programmers can use without surprises. 431 From the Library of Hristo Dimov Hristov 432 Chapter 8 Generic Programming The introduction of generics in Java 5 constitutes the most significant change in the Java programming language since its initial release. A major design goal was to be backwards compatible with earlier releases. As a result, Java generics have some uncomfortable limitations. You will learn about the benefits and challenges of generic programming in this chapter. 8.1 Why Generic Programming? Generic programming means writing code that can be reused for objects of many different types. For example, you don’t want to program separate classes to collect String and File objects. And you don’t have to—the single class ArrayList collects objects of any class. This is one example of generic programming. Actually, Java had an ArrayList class before it had generic classes. Let us investigate how the mechanism for generic programming has evolved, and what that means for users and implementors. 8.1.1 The Advantage of Type Parameters Before generic classes were added to Java, generic programming was achieved with inheritance. The ArrayList class simply maintained an array of Object references: public class ArrayList // before generic classes { private Object[] elementData; . . . public Object get(int i) { . . . } public void add(Object o) { . . . } } This approach has two problems. A cast is necessary whenever you retrieve a value: ArrayList files = new ArrayList(); . . . String filename = (String) files.get(0); Moreover, there is no error checking. You can add values of any class: files.add(new File(". . .")); This call compiles and runs without error. Elsewhere, casting the result of get to a String will cause an error. From the Library of Hristo Dimov Hristov 8.1 Why Generic Programming? 433 Generics offer a better solution: type parameters. The ArrayList class now has a type parameter that indicates the element type: var files = new ArrayList<String>(); This makes your code easier to read. You can tell right away that this particular array list contains String objects. NOTE: If you declare a variable with an explicit type instead of var, you can omit the type parameter in the constructor by using the “diamond” syntax: ArrayList<String> files = new ArrayList<>(); The omitted type is inferred from the type of the variable. Java 9 expands the use of the diamond syntax to situations where it was previously not accepted. For example, you can now use diamonds with anonymous subclasses: ArrayList<String> passwords = new ArrayList<>() // diamond OK in Java 9 { public String get(int n) { return super.get(n).replaceAll(".", "*"); } }; The compiler can make good use of the type information too. No cast is required for calling get. The compiler knows that the return type is String, not Object: String filename = files.get(0); The compiler also knows that the add method of an ArrayList<String> has a parameter of type String. That is a lot safer than having an Object parameter. Now the compiler can check that you don’t insert objects of the wrong type. For example, the statement files.add(new File(". . .")); // can only add String objects to an ArrayList<String> will not compile. A compiler error is much better than a class cast exception at runtime. This is the appeal of type parameters: They make your programs easier to read and safer. 8.1.2 Who Wants to Be a Generic Programmer? It is easy to use a generic class such as ArrayList. Most Java programmers will simply use types such as ArrayList<String> as if they had been built into the From the Library of Hristo Dimov Hristov 434 Chapter 8 Generic Programming language, just like String[] arrays. (Of course, array lists are better than arrays because they can expand automatically.) However, it is not so easy to implement a generic class. The programmers who use your code will want to plug in all sorts of classes for your type parameters. They will expect everything to work without onerous restrictions and confusing error messages. Your job as a generic programmer, therefore, is to anticipate all the potential future uses of your class. How hard can this get? Here is a typical issue that the designers of the standard class library had to grapple with. The ArrayList class has a method addAll to add all elements of another collection. A programmer may want to add all elements from an ArrayList<Manager> to an ArrayList<Employee>. But, of course, doing it the other way round should not be legal. How do you allow one call and disallow the other? The Java language designers invented an ingenious new concept, the wildcard type, to solve this problem. Wildcard types are rather abstract, but they allow a library builder to make methods as flexible as possible. Generic programming falls into three skill levels. At a basic level, you just use generic classes—typically, collections such as ArrayList—without thinking how and why they work. Most application programmers will want to stay at that level until something goes wrong. You may, however, encounter a confusing error message when mixing different generic classes, or when interfacing with legacy code that knows nothing about type parameters; at that point, you’ll need to learn enough about Java generics to solve problems systematically rather than through random tinkering. Finally, of course, you may want to implement your own generic classes and methods. Application programmers probably won’t write lots of generic code. The JDK developers have already done the heavy lifting and supplied type parameters for all the collection classes. As a rule of thumb, only code that traditionally involved lots of casts from very general types (such as Object or the Comparable interface) will benefit from using type parameters. In this chapter, we will show you everything you need to know to implement your own generic code. However, we expect that most readers will use this knowledge primarily for help with troubleshooting and to satisfy their curiosity about the inner workings of the parameterized collection classes. 8.2 Defining a Simple Generic Class A generic class is a class with one or more type variables. In this chapter, we will use a simple Pair class as an example. This class allows us to focus on From the Library of Hristo Dimov Hristov 8.2 Defining a Simple Generic Class 435 generics without being distracted by data storage details. Here is the code for the generic Pair class: public class Pair<T> { private T first; private T second; public Pair() { first = null; second = null; } public Pair(T first, T second) { this.first = first; this.second = second; } public T getFirst() { return first; } public T getSecond() { return second; } public void setFirst(T newValue) { first = newValue; } public void setSecond(T newValue) { second = newValue; } } The Pair class introduces a type variable T, enclosed in angle brackets < >, after the class name. A generic class can have more than one type variable. For example, we could have defined the Pair class with separate types for the first and second field: public class Pair<T, U> { . . . } The type variables are used throughout the class definition to specify method return types and the types of fields and local variables. For example: private T first; // uses the type variable NOTE: It is common practice to use uppercase letters for type variables, and to keep them short. The Java library uses the variable E for the element type of a collection, K and V for key and value types of a table, and T (and the neighboring letters U and S, if necessary) for “any type at all.” You instantiate the generic type by substituting types for the type variables, such as Pair<String> You can think of the result as an ordinary class with constructors Pair<String>() Pair<String>(String, String) and methods String getFirst() String getSecond() From the Library of Hristo Dimov Hristov 436 Chapter 8 Generic Programming void setFirst(String) void setSecond(String) In other words, the generic class acts as a factory for ordinary classes. The program in Listing 8.1 puts the Pair class to work. The static minmax method traverses an array and simultaneously computes the minimum and maximum values. It uses a Pair object to return both results. Recall that the compareTo method compares two strings, returning 0 if the strings are identical, a negative integer if the first string comes before the second in dictionary order, and a positive integer otherwise. C++ NOTE: Superficially, generic classes in Java are similar to template classes in C++. The only obvious difference is that Java has no special template keyword. However, as you will see throughout this chapter, there are substantial differences between these two mechanisms. Listing 8.1 pair1/PairTest1.java 1 package pair1; 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 /** * @version 1.01 2012-01-26 * @author Cay Horstmann */ public class PairTest1 { public static void main(String[] args) { String[] words = { "Mary", "had", "a", "little", "lamb" }; Pair<String> mm = ArrayAlg.minmax(words); System.out.println("min = " + mm.getFirst()); System.out.println("max = " + mm.getSecond()); } } 17 18 19 20 21 22 23 24 25 26 27 28 class ArrayAlg { /** * Gets the minimum and maximum of an array of strings. * @param a an array of strings * @return a pair with the min and max values, or null if a is null or empty */ public static Pair<String> minmax(String[] a) { if (a == null || a.length == 0) return null; String min = a[0]; From the Library of Hristo Dimov Hristov 8.3 437 String max = a[0]; for (int i = 1; i < a.length; i++) { if (min.compareTo(a[i]) > 0) min = a[i]; if (max.compareTo(a[i]) < 0) max = a[i]; } return new Pair<>(min, max); 29 30 31 32 33 34 35 } 36 37 Generic Methods } 8.3 Generic Methods In the preceding section, you have seen how to define a generic class. You can also define a single method with type parameters. class ArrayAlg { public static <T> T getMiddle(T... a) { return a[a.length / 2]; } } This method is defined inside an ordinary class, not inside a generic class. However, it is a generic method, as you can see from the angle brackets and the type variable. Note that the type variables are inserted after the modifiers (public static, in our case) and before the return type. You can define generic methods both inside ordinary classes and inside generic classes. When you call a generic method, you can place the actual types, enclosed in angle brackets, before the method name: String middle = ArrayAlg.<String>getMiddle("John", "Q.", "Public"); In this case (and indeed in most cases), you can omit the <String> type parameter from the method call. The compiler has enough information to infer the method that you want. It matches the type of the arguments against the generic type T... and deduces that T must be String. That is, you can simply call String middle = ArrayAlg.getMiddle("John", "Q.", "Public"); In almost all cases, type inference for generic methods works smoothly. Occasionally, the compiler gets it wrong, and you’ll need to decipher an error report. Consider this example: double middle = ArrayAlg.getMiddle(3.14, 1729, 0); From the Library of Hristo Dimov Hristov 438 Chapter 8 Generic Programming The error message complains, in cryptic terms that vary from one compiler version to another, that there are two ways of interpreting this code, both equally valid. In a nutshell, the compiler autoboxed the parameters into a Double and two Integer objects, and then it tried to find a common supertype of these classes. It actually found two: Number and the Comparable interface, which is itself a generic type. In this case, the remedy is to write all parameters as double values. TIP: Peter von der Ahé recommends this trick if you want to see which type the compiler infers for a generic method call: Purposefully introduce an error and study the resulting error message. For example, consider the call ArrayAlg.getMiddle("Hello", 0, null). Assign the result to a JButton, which can’t possibly be right. You will get an error report: found: java.lang.Object&java.io.Serializable&java.lang.Comparable<? extends java.lang.Object&java.io.Serializable&java.lang.Comparable<?>> In plain English, you can assign the result to Object, Serializable, or Comparable. C++ NOTE: In C++, you place the type parameters after the method name. That can lead to nasty parsing ambiguities. For example, g(f<a,b>(c)) can mean “call g with the result of f<a,b>(c)”, or “call g with the two boolean values f<a and b>(c)”. 8.4 Bounds for Type Variables Sometimes, a class or a method needs to place restrictions on type variables. Here is a typical example. We want to compute the smallest element of an array: class ArrayAlg { public static <T> T min(T[] a) // almost correct { if (a == null || a.length == 0) return null; T smallest = a[0]; for (int i = 1; i < a.length; i++) if (smallest.compareTo(a[i]) > 0) smallest = a[i]; return smallest; } } From the Library of Hristo Dimov Hristov 8.4 Bounds for Type Variables 439 But there is a problem. Look inside the code of the min method. The variable smallest has type T, which means it could be an object of an arbitrary class. How do we know that the class to which T belongs has a compareTo method? The solution is to restrict T to a class that implements the Comparable interface—a standard interface with a single method, compareTo. You can achieve this by giving a bound for the type variable T: public static <T extends Comparable> T min(T[] a) . . . Actually, the Comparable interface is itself a generic type. For now, we will ignore that complexity and the warnings that the compiler generates. Section 8.8, “Wildcard Types,” on p. 459 discusses how to properly use type parameters with the Comparable interface. Now, the generic min method can only be called with arrays of classes that implement the Comparable interface, such as String, LocalDate, and so on. Calling min with a Rectangle array is a compile-time error because the Rectangle class does not implement Comparable. C++ NOTE: In C++, you cannot restrict the types of template parameters. If a programmer instantiates a template with an inappropriate type, an (often obscure) error message is reported inside the template code. You may wonder why we use the extends keyword rather than the implements keyword in this situation—after all, Comparable is an interface. The notation <T extends BoundingType> expresses that T should be a subtype of the bounding type. Both T and the bounding type can be either a class or an interface. The extends keyword was chosen because it is a reasonable approximation of the subtype concept, and the Java designers did not want to add a new keyword (such as sub) to the language. A type variable or wildcard can have multiple bounds. For example: T extends Comparable & Serializable The bounding types are separated by ampersands (&) because commas are used to separate type variables. As with Java inheritance, you can have as many interface supertypes as you like, but at most one of the bounds can be a class. If you have a class as a bound, it must be the first one in the bounds list. From the Library of Hristo Dimov Hristov 440 Chapter 8 Generic Programming In the next sample program (Listing 8.2), we rewrite the minmax method to be generic. The method computes the minimum and maximum of a generic array, returning a Pair<T>. Listing 8.2 pair2/PairTest2.java 1 package pair2; 2 3 import java.time.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 /** * @version 1.02 2015-06-21 * @author Cay Horstmann */ public class PairTest2 { public static void main(String[] args) { LocalDate[] birthdays = { LocalDate.of(1906, 12, 9), // G. Hopper LocalDate.of(1815, 12, 10), // A. Lovelace LocalDate.of(1903, 12, 3), // J. von Neumann LocalDate.of(1910, 6, 22), // K. Zuse }; Pair<LocalDate> mm = ArrayAlg.minmax(birthdays); System.out.println("min = " + mm.getFirst()); System.out.println("max = " + mm.getSecond()); } } 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 class ArrayAlg { /** Gets the minimum and maximum of an array of objects of type T. @param a an array of objects of type T @return a pair with the min and max values, or null if a is null or empty */ public static <T extends Comparable> Pair<T> minmax(T[] a) { if (a == null || a.length == 0) return null; T min = a[0]; T max = a[0]; for (int i = 1; i < a.length; i++) { if (min.compareTo(a[i]) > 0) min = a[i]; if (max.compareTo(a[i]) < 0) max = a[i]; } From the Library of Hristo Dimov Hristov 8.5 441 return new Pair<>(min, max); 43 } 44 45 Generic Code and the Virtual Machine } 8.5 Generic Code and the Virtual Machine The virtual machine does not have objects of generic types—all objects belong to ordinary classes. An earlier version of the generics implementation was even able to compile a program that used generics into class files that executed on 1.0 virtual machines! In the following sections, you will see how the compiler “erases” type parameters, and what implication that process has for Java programmers. 8.5.1 Type Erasure Whenever you define a generic type, a corresponding raw type is automatically provided. The name of the raw type is simply the name of the generic type, with the type parameters removed. The type variables are erased and replaced by their bounding types (or Object for variables without bounds). For example, the raw type for Pair<T> looks like this: public class Pair { private Object first; private Object second; public Pair(Object first, Object second) { this.first = first; this.second = second; } public Object getFirst() { return first; } public Object getSecond() { return second; } public void setFirst(Object newValue) { first = newValue; } public void setSecond(Object newValue) { second = newValue; } } Since T is an unbounded type variable, it is simply replaced by Object. The result is an ordinary class, just as you might have implemented it before generics were added to Java. Your programs may contain different kinds of Pair, such as Pair<String> or Pair<LocalDate>, but erasure turns them all into raw Pair types. From the Library of Hristo Dimov Hristov 442 Chapter 8 Generic Programming C++ NOTE: In this regard, Java generics are very different from C++ templates. C++ produces different types for each template instantiation—a phenomenon called “template code bloat.” Java does not suffer from this problem. The raw type replaces type variables with the first bound, or Object if no bounds are given. For example, the type variable in the class Pair<T> has no explicit bounds, hence the raw type replaces T with Object. Suppose we declare a slightly different type: public class Interval<T extends Comparable & Serializable> implements Serializable { private T lower; private T upper; . . . public Interval(T first, T second) { if (first.compareTo(second) <= 0) { lower = first; upper = second; } else { lower = second; upper = first; } } } The raw type Interval looks like this: public class Interval implements Serializable { private Comparable lower; private Comparable upper; . . . public Interval(Comparable first, Comparable second) { . . . } } NOTE: You may wonder what happens if you switch the bounds: class Interval<T extends Serializable & Comparable>. In that case, the raw type replaces T with Serializable, and the compiler inserts casts to Comparable when necessary. For efficiency, you should therefore put tagging interfaces (that is, interfaces without methods) at the end of the bounds list. 8.5.2 Translating Generic Expressions When you program a call to a generic method, the compiler inserts casts when the return type has been erased. For example, consider the sequence of statements Pair<Employee> buddies = . . .; Employee buddy = buddies.getFirst(); From the Library of Hristo Dimov Hristov 8.5 Generic Code and the Virtual Machine 443 The erasure of getFirst has return type Object. The compiler automatically inserts the cast to Employee. That is, the compiler translates the method call into two virtual machine instructions: • A call to the raw method Pair.getFirst • A cast of the returned Object to the type Employee Casts are also inserted when you access a generic field. Suppose the first and second fields of the Pair class were public. (Not a good programming style, perhaps, but it is legal Java.) Then the expression Employee buddy = buddies.first; also has a cast inserted in the resulting bytecodes. 8.5.3 Translating Generic Methods Type erasure also happens for generic methods. Programmers usually think of a generic method such as public static <T extends Comparable> T min(T[] a) as a whole family of methods, but after erasure, only a single method is left: public static Comparable min(Comparable[] a) Note that the type parameter T has been erased, leaving only its bounding type Comparable. Erasure of methods brings up a couple of complexities. Consider this example: class DateInterval extends Pair<LocalDate> { public void setSecond(LocalDate second) { if (second.compareTo(getFirst()) >= 0) super.setSecond(second); } . . . } A date interval is a pair of LocalDate objects, and we’ll want to override the methods to ensure that the second value is never smaller than the first. This class is erased to class DateInterval extends Pair // after erasure { public void setSecond(LocalDate second) { . . . } . . . } From the Library of Hristo Dimov Hristov 444 Chapter 8 Generic Programming Perhaps surprisingly, there is another setSecond method, inherited from Pair, namely public void setSecond(Object second) This is clearly a different method because it has a parameter of a different type—Object instead of LocalDate. But it shouldn’t be different. Consider this sequence of statements: var interval = new DateInterval(. . .); Pair<LocalDate> pair = interval; // OK--assignment to superclass pair.setSecond(aDate); Our expectation is that the call to setSecond is polymorphic and that the appropriate method is called. Since pair refers to a DateInterval object, that should be DateInterval.setSecond. The problem is that the type erasure interferes with polymorphism. To fix this problem, the compiler generates a bridge method in the DateInterval class: public void setSecond(Object second) { setSecond((LocalDate) second); } To see why this works, let us carefully follow the execution of the statement pair.setSecond(aDate) The variable pair has declared type Pair<LocalDate>, and that type only has a single method called setSecond, namely setSecond(Object). The virtual machine calls that method on the object to which pair refers. That object is of type DateInterval. Therefore, the method DateInterval.setSecond(Object) is called. That method is the synthesized bridge method. It calls DateInterval.setSecond(LocalDate), which is what we want. Bridge methods can get even stranger. Suppose the DateInterval class also overrides the getSecond method: class DateInterval extends Pair<LocalDate> { public LocalDate getSecond() { return (LocalDate) super.getSecond(); } . . . } In the DateInterval class, there are two getSecond methods: LocalDate getSecond() // defined in DateInterval Object getSecond() // overrides the method defined in Pair to call the first method You could not write Java code like that; it would be illegal to have two methods with the same parameter types—here, with no parameters. However, in the virtual machine, the parameter types and the return type specify a method. Therefore, the compiler can produce bytecodes for two methods that differ From the Library of Hristo Dimov Hristov 8.5 Generic Code and the Virtual Machine 445 only in their return type, and the virtual machine will handle this situation correctly. NOTE: Bridge methods are not limited to generic types. We already noted in Chapter 5 that it is legal for a method to specify a more restrictive return type when overriding another method. For example: public class Employee implements Cloneable { public Employee clone() throws CloneNotSupportedException { . . . } } The Object.clone and Employee.clone methods are said to have covariant return types. Actually, the Employee class has two clone methods: Employee clone() // defined above Object clone() // synthesized bridge method, overrides Object.clone The synthesized bridge method calls the newly defined method. In summary, you need to remember these facts about translation of Java generics: • There are no generics in the virtual machine, only ordinary classes and methods. • All type parameters are replaced by their bounds. • Bridge methods are synthesized to preserve polymorphism. • Casts are inserted as necessary to preserve type safety. 8.5.4 Calling Legacy Code When Java generics were designed, a major goal was to allow interoperability between generics and legacy code. Let us look at a concrete example of such legacy. The Swing user interface toolkit provides a JSlider class whose “ticks” can be customized with labels that contain text or images. The labels are set with the call void setLabelTable(Dictionary table) The Dictionary class maps integers to labels. Before Java 5, that class was implemented as a map of Object instances. Java 5 made Dictionary into a generic class, but JSlider was never updated. At this point, Dictionary without type parameters is a raw type. This is where compatibility comes in. From the Library of Hristo Dimov Hristov 446 Chapter 8 Generic Programming When you populate the dictionary, you can use the generic type. Dictionary<Integer, Component> labelTable = new Hashtable<>(); labelTable.put(0, new JLabel(new ImageIcon("nine.gif"))); labelTable.put(20, new JLabel(new ImageIcon("ten.gif"))); . . . When you pass the Dictionary<Integer, Component> object to setLabelTable, the compiler issues a warning. slider.setLabelTable(labelTable); // warning After all, the compiler has no assurance about what the setLabelTable might do to the Dictionary object. That method might replace all the keys with strings. That breaks the guarantee that the keys have type Integer, and future operations may cause bad cast exceptions. You should ponder it and ask what the JSlider is actually going to do with this Dictionary object. In our case, it is pretty clear that the JSlider only reads the information, so we can ignore the warning. Now consider the opposite case, in which you get an object of a raw type from a legacy class. You can assign it to a variable whose type uses generics, but of course you will get a warning. For example: Dictionary<Integer, Components> labelTable = slider.getLabelTable(); // warning That’s OK—review the warning and make sure that the label table really contains Integer and Component objects. Of course, there never is an absolute guarantee. A malicious coder might have installed a different Dictionary in the slider. But again, the situation is no worse than it was before generics. In the worst case, your program will throw an exception. After you are done pondering the warning, you can use an annotation to make it disappear. You can annotate a local variable: @SuppressWarnings("unchecked") Dictionary<Integer, Components> labelTable = slider.getLabelTable(); // no warning Or you can annotate an entire method, like this: @SuppressWarnings("unchecked") public void configureSlider() { . . . } This annotation turns off checking for all code inside the method. From the Library of Hristo Dimov Hristov 8.6 Restrictions and Limitations 447 8.6 Restrictions and Limitations In the following sections, we discuss a number of restrictions that you need to consider when working with Java generics. Most of these restrictions are a consequence of type erasure. 8.6.1 Type Parameters Cannot Be Instantiated with Primitive Types You cannot substitute a primitive type for a type parameter. Thus, there is no Pair<double>, only Pair<Double>. The reason is, of course, type erasure. After erasure, the Pair class has fields of type Object, and you can’t use them to store double values. This is an annoyance, to be sure, but it is consistent with the separate status of primitive types in the Java language. It is not a fatal flaw—there are only eight primitive types, and you can always handle them with separate classes and methods when wrapper types are not an acceptable substitute. 8.6.2 Runtime Type Inquiry Only Works with Raw Types Objects in the virtual machine always have a specific nongeneric type. Therefore, all type inquiries yield only the raw type. For example, if (a instanceof Pair<String>) // ERROR could only test whether a is a Pair of any type. The same is true for the test if (a instanceof Pair<T>) // ERROR or the cast Pair<String> p = (Pair<String>) a; // warning--can only test that a is a Pair To remind you of the risk, you will get a compiler error (with instanceof) or warning (with casts) when you try to inquire whether an object belongs to a generic type. In the same spirit, the getClass method always returns the raw type. For example: Pair<String> stringPair = . . .; Pair<Employee> employeePair = . . .; if (stringPair.getClass() == employeePair.getClass()) // they are equal The comparison yields true because both calls to getClass return Pair.class. From the Library of Hristo Dimov Hristov 448 Chapter 8 Generic Programming 8.6.3 You Cannot Create Arrays of Parameterized Types You cannot instantiate arrays of parameterized types, such as var table = new Pair<String>[10]; // ERROR What’s wrong with that? After erasure, the type of table is Pair[]. You can convert it to Object[]: Object[] objarray = table; An array remembers its component type and throws an ArrayStoreException if you try to store an element of the wrong type: objarray[0] = "Hello"; // ERROR--component type is Pair But erasure renders this mechanism ineffective for generic types. The assignment objarray[0] = new Pair<Employee>(); would pass the array store check but still result in a type error. For this reason, arrays of parameterized types are outlawed. Note that only the creation of these arrays is outlawed. You can declare a variable of type Pair<String>[]. But you can’t initialize it with a new Pair<String>[10]. NOTE: You can declare arrays of wildcard types and then cast them: var table = (Pair<String>[]) new Pair<?>[10]; The result is not safe. If you store a Pair<Employee> in table[0] and then call a String method on table[0].getFirst(), you get a ClassCastException. TIP: If you need to collect parameterized type objects, simply use an ArrayList: ArrayList<Pair<String>> is safe and effective. 8.6.4 Varargs Warnings In the preceding section, you saw that Java doesn’t support arrays of generic types. In this section, we discuss a related issue: passing instances of a generic type to a method with a variable number of arguments. Consider this simple method with variable arguments: From the Library of Hristo Dimov Hristov 8.6 Restrictions and Limitations 449 public static <T> void addAll(Collection<T> coll, T... ts) { for (T t : ts) coll.add(t); } Recall that the parameter ts is actually an array that holds all supplied arguments. Now consider this call: Collection<Pair<String>> table = . . .; Pair<String> pair1 = . . .; Pair<String> pair2 = . . .; addAll(table, pair1, pair2); In order to call this method, the Java virtual machine must make an array of Pair<String>, which is against the rules. However, the rules have been relaxed for this situation, and you only get a warning, not an error. You can suppress the warning in one of two ways. You can add the annotation @SuppressWarnings("unchecked") to the method containing the call to addAll. Or, as of Java 7, you can annotate the addAll method itself with @SafeVarargs: @SafeVarargs public static <T> void addAll(Collection<T> coll, T... ts) This method can now be called with generic types. You can use this annotation for any methods that merely read the elements of the parameter array, which is bound to be the most common use case. The @SafeVarargs can only be used with constructors and methods that are static, final, or (as of Java 9) private. Any other method could be overridden, making the annotation meaningless. NOTE: You can use the @SafeVarargs annotation to defeat the restriction against generic array creation, using this method: @SafeVarargs static <E> E[] array(E... array) { return array; } Now you can call Pair<String>[] table = array(pair1, pair2); This seems convenient, but there is a hidden danger. The code Object[] objarray = table; objarray[0] = new Pair<Employee>(); will run without an ArrayStoreException (because the array store only checks the erased type), and you’ll get an exception elsewhere when you work with table[0]. From the Library of Hristo Dimov Hristov 450 Chapter 8 Generic Programming 8.6.5 You Cannot Instantiate Type Variables You cannot use type variables in an expression such as new T(. . .). For example, the following Pair<T> constructor is illegal: public Pair() { first = new T(); second = new T(); } // ERROR Type erasure would change T to Object, and surely you don’t want to call new Object(). The best workaround, available since Java 8, is to make the caller provide a constructor expression. For example: Pair<String> p = Pair.makePair(String::new); The makePair method receives a Supplier<T>, the functional interface for a function with no arguments and a result of type T: public static <T> Pair<T> makePair(Supplier<T> constr) { return new Pair<>(constr.get(), constr.get()); } A more traditional workaround is to construct generic objects through reflection, by calling the Constructor.newInstance method. Unfortunately, the details are a bit complex. You cannot call first = T.class.getConstructor().newInstance(); // ERROR The expression T.class is not legal because it would erase to Object.class. Instead, you must design the API so that you are handed a Class object, like this: public static <T> Pair<T> makePair(Class<T> cl) { try { return new Pair<>(cl.getConstructor().newInstance(), cl.getConstructor().newInstance()); } catch (Exception e) { return null; } } This method could be called as follows: Pair<String> p = Pair.makePair(String.class); Note that the Class class is itself generic. For example, String.class is an instance (indeed, the sole instance) of Class<String>. Therefore, the makePair method can infer the type of the pair that it is making. From the Library of Hristo Dimov Hristov 8.6 Restrictions and Limitations 451 8.6.6 You Cannot Construct a Generic Array Just as you cannot instantiate a single generic instance, you cannot instantiate an array. The reasons are different—an array is, after all, filled with null values, which would seem safe to construct. But an array also carries a type, which is used to monitor array stores in the virtual machine. That type is erased. For example, consider public static <T extends Comparable> T[] minmax(T... a) { T[] mm = new T[2]; // ERROR . . . } Type erasure would cause this method to always construct an array Comparable[2]. If the array is only used as a private instance field of a class, you can declare the element type of the array to be the erased type and use casts. For example, the ArrayList class could be implemented as follows: public class ArrayList<E> { private Object[] elements; . . . @SuppressWarnings("unchecked") public E get(int n) { return (E) elements[n]; } public void set(int n, E e) { elements[n] = e; } // no cast needed } The actual implementation is not quite as clean: public class ArrayList<E> { private E[] elements; . . . public ArrayList() { elements = (E[]) new Object[10]; } } Here, the cast E[] is an outright lie, but type erasure makes it undetectable. This technique does not work for our minmax method since we are returning a array, and a runtime error results if we lie about its type. Suppose we implement T[] public static <T extends Comparable> T[] minmax(T... a) { var result = new Comparable[2]; // array of erased type . . . return (T[]) result; // compiles with warning } The call From the Library of Hristo Dimov Hristov 452 Chapter 8 Generic Programming String[] names = ArrayAlg.minmax("Tom", "Dick", "Harry"); compiles without any warning. A ClassCastException occurs when the Comparable[] reference is cast to String[] after the method returns. In this situation, it is best to ask the user to provide an array constructor expression: String[] names = ArrayAlg.minmax(String[]::new, "Tom", "Dick", "Harry"); The constructor expression String[]::new denotes a function that, given the desired length, constructs a String array of that length. The method uses that parameter to produce an array of the correct type: public static <T extends Comparable> T[] minmax(IntFunction<T[]> constr, T... a) { T[] result = constr.apply(2); . . . } A more old-fashioned approach is to use reflection and call Array.newInstance: public static <T extends Comparable> T[] minmax(T... a) { var result = (T[]) Array.newInstance(a.getClass().getComponentType(), 2); . . . } The toArray method of the ArrayList class is not so lucky. It needs to produce a T[] array, but it doesn’t have the component type. Therefore, there are two variants: Object[] toArray() T[] toArray(T[] result) The second method receives an array parameter. If the array is large enough, it is used. Otherwise, a new array of sufficient size is created, using the component type of result. 8.6.7 Type Variables Are Not Valid in Static Contexts of Generic Classes You cannot reference type variables in static fields or methods. For example, the following clever idea won’t work: public class Singleton<T> { private static T singleInstance; // ERROR public static T getSingleInstance() // ERROR { From the Library of Hristo Dimov Hristov 8.6 Restrictions and Limitations 453 if (singleInstance == null) construct new instance of T return singleInstance; } } If this could be done, then a program could declare a Singleton<Random> to share a random number generator and a Singleton<JFileChooser> to share a file chooser dialog. But it can’t work. After type erasure there is only one Singleton class, and only one singleInstance field. For that reason, static fields and methods with type variables are simply outlawed. 8.6.8 You Cannot Throw or Catch Instances of a Generic Class You can neither throw nor catch objects of a generic class. In fact, it is not even legal for a generic class to extend Throwable. For example, the following definition will not compile: public class Problem<T> extends Exception { /* . . . */ } // ERROR--can't extend Throwable You cannot use a type variable in a catch clause. For example, the following method will not compile: public static <T extends Throwable> void doWork(Class<T> t) { try { do work } catch (T e) // ERROR--can't catch type variable { Logger.getGlobal().info(. . .); } } However, it is OK to use type variables in exception specifications. The following method is legal: public static <T extends Throwable> void doWork(T t) throws T // OK { try { do work } catch (Throwable realCause) { t.initCause(realCause); throw t; } } From the Library of Hristo Dimov Hristov 454 Chapter 8 Generic Programming 8.6.9 You Can Defeat Checked Exception Checking A bedrock principle of Java exception handling is that you must provide a handler for all checked exceptions. You can use generics to defeat this scheme. The key ingredient is this method: @SuppressWarnings("unchecked") static <T extends Throwable> void throwAs(Throwable t) throws T { throw (T) t; } Suppose this method is contained in an interface Task. When you have a checked exception e and call Task.<RuntimeException>throwAs(e); then the compiler will believe that e becomes an unchecked exception. The following turns all exceptions into those that the compiler believes to be unchecked: try { do work } catch (Throwable t) { Task.<RuntimeException>throwAs(t); } Let’s use this to solve a vexing problem. To run code in a thread, you have to place it into the run method of a class that implements the Runnable interface. But that method is not allowed to throw checked exceptions. We will provide an adaptor from a Task, whose run method is allowed to throw arbitrary exceptions, to a Runnable: interface Task { void run() throws Exception; @SuppressWarnings("unchecked") static <T extends Throwable> void throwAs(Throwable t) throws T { throw (T) t; } static Runnable asRunnable(Task task) { return () -> { From the Library of Hristo Dimov Hristov 8.6 Restrictions and Limitations 455 try { task.run(); } catch (Exception e) { Task.<RuntimeException>throwAs(e); } }; } } For example, this program runs a thread that will throw a checked exception: public class Test { public static void main(String[] args) { var thread = new Thread(Task.asRunnable(() -> { Thread.sleep(1000); System.out.println("Hello, World!"); throw new Exception("Check this out!"); })); thread.start(); } } The Thread.sleep method is declared to throw an InterruptedException, and we no longer have to catch it. Since we don’t interrupt the thread, that exception won’t be thrown. However, the program throws a checked exception. When you run the program, you will get a stack trace. What’s so remarkable about that? Normally, you have to catch all checked exceptions inside the run method of a Runnable and wrap them into unchecked exceptions—the run method is declared to throw no checked exceptions. But here, we don’t wrap. We simply throw the exception, tricking the compiler into believing that it is not a checked exception. Using generic classes, erasure, and the @SuppressWarnings annotation, we were able to defeat an essential part of the Java type system. 8.6.10 Beware of Clashes after Erasure It is illegal to create conditions that cause clashes when generic types are erased. Here is an example. Suppose we add an equals method to the Pair class, like this: From the Library of Hristo Dimov Hristov 456 Chapter 8 Generic Programming public class Pair<T> { public boolean equals(T value) { return first.equals(value) && second.equals(value); } . . . } Consider a Pair<String>. Conceptually, it has two equals methods: boolean equals(String) // defined in Pair<T> boolean equals(Object) // inherited from Object But the intuition leads us astray. The erasure of the method boolean equals(T) is boolean equals(Object) which clashes with the Object.equals method. The remedy is, of course, to rename the offending method. The generics specification cites another rule: “To support translation by erasure, we impose the restriction that a class or type variable may not at the same time be a subtype of two interface types which are different parameterizations of the same interface.” For example, the following is illegal: class Employee implements Comparable<Employee> { . . . } class Manager extends Employee implements Comparable<Manager> { . . . } // ERROR Manager would then implement both Comparable<Employee> and Comparable<Manager>, which are different parameterizations of the same interface. It is not obvious what this restriction has to do with type erasure. After all, the nongeneric version class Employee implements Comparable { . . . } class Manager extends Employee implements Comparable { . . . } is legal. The reason is far more subtle. There would be a conflict with the synthesized bridge methods. A class that implements Comparable<X> gets a bridge method public int compareTo(Object other) { return compareTo((X) other); } You cannot have two such methods for different types X. From the Library of Hristo Dimov Hristov 8.7 Inheritance Rules for Generic Types 457 8.7 Inheritance Rules for Generic Types When you work with generic classes, you need to learn a few rules about inheritance and subtypes. Let’s start with a situation which many programmers find unintuitive. Consider a class and a subclass, such as Employee and Manager. Is Pair<Manager> a subclass of Pair<Employee>? Perhaps surprisingly, the answer is “no.” For example, the following code will not compile: Manager[] topHonchos = . . .; Pair<Employee> result = ArrayAlg.minmax(topHonchos); // ERROR The minmax method returns a Pair<Manager>, not a Pair<Employee>, and it is illegal to assign one to the other. In general, there is no relationship between Pair<S> and Pair<T>, no matter how S and T are related (see Figure 8.1). Figure 8.1 No inheritance relationship between pair classes This seems like a cruel restriction, but it is necessary for type safety. Suppose we were allowed to convert a Pair<Manager> to a Pair<Employee>. Consider this code: var managerBuddies = new Pair<Manager>(ceo, cfo); Pair<Employee> employeeBuddies = managerBuddies; // illegal, but suppose it wasn't employeeBuddies.setFirst(lowlyEmployee); Clearly, the last statement is legal. But employeeBuddies and managerBuddies refer to the same object. We now managed to pair up the CFO with a lowly employee, which should not be possible for a Pair<Manager>. From the Library of Hristo Dimov Hristov 458 Chapter 8 Generic Programming NOTE: You just saw an important difference between generic types and Java arrays. You can assign a Manager[] array to a variable of type Employee[]: Manager[] managerBuddies = { ceo, cfo }; Employee[] employeeBuddies = managerBuddies; // OK However, arrays come with special protection. If you try to store a lowly employee into employeeBuddies[0], the virtual machine throws an ArrayStoreException. You can always convert a parameterized type to a raw type. For example, Pair<Employee> is a subtype of the raw type Pair. This conversion is necessary for interfacing with legacy code. Figure 8.2 Subtype relationships among generic list types From the Library of Hristo Dimov Hristov 8.8 Wildcard Types 459 Can you convert to the raw type and then cause a type error? Unfortunately, you can. Consider this example: var managerBuddies = new Pair<Manager>(ceo, cfo); Pair rawBuddies = managerBuddies; // OK rawBuddies.setFirst(new File(". . .")); // only a compile-time warning This sounds scary. However, keep in mind that you are no worse off than you were with older versions of Java. The security of the virtual machine is not at stake. When the foreign object is retrieved with getFirst and assigned to a Manager variable, a ClassCastException is thrown, just as in the good old days. You merely lose the added safety that generic programming normally provides. Finally, generic classes can extend or implement other generic classes. In this regard, they are no different from ordinary classes. For example, the class ArrayList<T> implements the interface List<T>. That means an ArrayList<Manager> can be converted to a List<Manager>. However, as you just saw, an ArrayList<Manager> is not an ArrayList<Employee> or List<Employee>. Figure 8.2 shows these relationships. 8.8 Wildcard Types It was known for some time among researchers of type systems that a rigid system of generic types is quite unpleasant to use. The Java designers invented an ingenious (but nevertheless safe) “escape hatch”: the wildcard type. The following sections show you how to work with wildcards. 8.8.1 The Wildcard Concept In a wildcard type, a type parameter is allowed to vary. For example, the wildcard type Pair<? extends Employee> denotes any generic Pair type whose type parameter is a subclass of Employee, such as Pair<Manager>, but not Pair<String>. Let’s say you want to write a method that prints out pairs of employees, like this: public static void printBuddies(Pair<Employee> p) { Employee first = p.getFirst(); Employee second = p.getSecond(); System.out.println(first.getName() + " and " + second.getName() + " are buddies."); } From the Library of Hristo Dimov Hristov 460 Chapter 8 Generic Programming As you saw in the preceding section, you cannot pass a Pair<Manager> to that method, which is rather limiting. But the solution is simple—use a wildcard type: public static void printBuddies(Pair<? extends Employee> p) The type Pair<Manager> is a subtype of Pair<? extends Employee> (see Figure 8.3). Figure 8.3 Subtype relationships with wildcards Can we use wildcards to corrupt a Pair<Manager> through a Pair<? extends Employee> reference? var managerBuddies = new Pair<Manager>(ceo, cfo); Pair<? extends Employee> wildcardBuddies = managerBuddies; // OK wildcardBuddies.setFirst(lowlyEmployee); // compile-time error No corruption is possible. The call to setFirst is a type error. To see why, let us have a closer look at the type Pair<? extends Employee>. Its methods look like this: ? extends Employee getFirst() void setFirst(? extends Employee) This makes it impossible to call the setFirst method. The compiler only knows that it needs some subtype of Employee, but it doesn’t know which type. It refuses to pass any specific type—after all, ? might not match it. From the Library of Hristo Dimov Hristov 8.8 Wildcard Types 461 We don’t have this problem with getFirst: It is perfectly legal to assign the return value of getFirst to an Employee reference. This is the key idea behind bounded wildcards. We now have a way of distinguishing between the safe accessor methods and the unsafe mutator methods. 8.8.2 Supertype Bounds for Wildcards Wildcard bounds are similar to type variable bounds, but they have an added capability—you can specify a supertype bound, like this: ? super Manager This wildcard is restricted to all supertypes of Manager. (It was a stroke of good luck that the existing super keyword describes the relationship so accurately.) Why would you want to do this? A wildcard with a supertype bound gives you a behavior that is opposite to that of the wildcards described in Section 8.8, “Wildcard Types,” on p. 459. You can supply parameters to methods, but you can’t use the return values. For example, Pair<? super Manager> has methods that can be described as follows: void setFirst(? super Manager) ? super Manager getFirst() This is not actual Java syntax, but it shows what the compiler knows. The compiler cannot know the exact type of the setFirst method and therefore cannot accept a call with an argument of type Employee or Object. It is only possible to pass an object of type Manager or a subtype such as Executive. Moreover, if you call getFirst, there is no guarantee about the type of the returned object. You can only assign it to an Object. Here is a typical example. We have an array of managers and want to put the manager with the lowest and highest bonus into a Pair object. What kind of Pair? A Pair<Employee> should be fair game or, for that matter, a Pair<Object> (see Figure 8.4). The following method will accept any appropriate Pair: public static void minmaxBonus(Manager[] a, Pair<? super Manager> result) { if (a.length == 0) return; Manager min = a[0]; Manager max = a[0]; for (int i = 1; i < a.length; i++) { if (min.getBonus() > a[i].getBonus()) min = a[i]; if (max.getBonus() < a[i].getBonus()) max = a[i]; } From the Library of Hristo Dimov Hristov 462 Chapter 8 Generic Programming Figure 8.4 A wildcard with a supertype bound result.setFirst(min); result.setSecond(max); } Intuitively speaking, wildcards with supertype bounds let you write to a generic object, while wildcards with subtype bounds let you read from a generic object. Here is another use for supertype bounds. The Comparable interface is itself a generic type. It is declared as follows: public interface Comparable<T> { public int compareTo(T other); } From the Library of Hristo Dimov Hristov 8.8 Wildcard Types 463 Here, the type variable indicates the type of the other parameter. For example, the String class implements Comparable<String>, and its compareTo method is declared as public int compareTo(String other) This is nice—the explicit parameter has the correct type. Before the interface was generic, other was an Object, and a cast was necessary in the implementation of the method. Now that Comparable is a generic type, perhaps we should have done a better job with the minmax method of the ArrayAlg class? We could have declared it as public static <T extends Comparable<T>> Pair<T> minmax(T[] a) This looks more thorough than just using T extends Comparable, and it would work fine for many classes. For example, if you compute the minimum of a String array, then T is the type String, and String is a subtype of Comparable<String>. But we run into a problem when processing an array of LocalDate objects. As it happens, LocalDate implements ChronoLocalDate, and ChronoLocalDate extends Comparable<ChronoLocalDate>. Thus, LocalDate implements Comparable<ChronoLocalDate> but not Comparable<LocalDate>. In a situation such as this one, supertypes come to the rescue: public static <T extends Comparable<? super T>> Pair<T> minmax(T[] a) Now the compareTo method has the form int compareTo(? super T) Maybe it is declared to take an object of type T, or—for example, when T is LocalDate—a supertype of T. At any rate, it is safe to pass an object of type T to the compareTo method. To the uninitiated, a declaration such as <T extends Comparable<? super T>> is bound to look intimidating. This is unfortunate, because the intent of this declaration is to help application programmers by removing unnecessary restrictions on the call parameters. Application programmers with no interest in generics will probably learn quickly to gloss over these declarations and just take for granted that library programmers will do the right thing. If you are a library programmer, you’ll need to get used to wildcards, or your users will curse you and throw random casts at their code until it compiles. From the Library of Hristo Dimov Hristov 464 Chapter 8 Generic Programming NOTE: Another common use for supertype bounds is an argument type of a functional interface. For example, the Collection interface has a method default boolean removeIf(Predicate<? super E> filter) The method removes all elements that fulfill the given predicate. For example, if you hate employees with odd hash codes, you can remove them like this: ArrayList<Employee> staff = . . .; Predicate<Object> oddHashCode = obj -> obj.hashCode() %2 != 0; staff.removeIf(oddHashCode); You want to be able to pass a Predicate<Object>, not just a Predicate<Employee>. The super wildcard makes that possible. 8.8.3 Unbounded Wildcards You can even use wildcards with no bounds at all—for example, Pair<?>. At first glance, this looks identical to the raw Pair type. Actually, the types are very different. The type Pair<?> has methods such as ? getFirst() void setFirst(?) The return value of getFirst can only be assigned to an Object. The setFirst method can never be called, not even with an Object. That’s the essential difference between Pair<?> and Pair: you can call the setFirst method of the raw Pair class with any Object. NOTE: You can call setFirst(null). Why would you ever want such a wimpy type? It is useful for very simple operations. For example, the following method tests whether a pair contains a null reference. It never needs the actual type. public static boolean hasNulls(Pair<?> p) { return p.getFirst() == null || p.getSecond() == null; } You could have avoided the wildcard type by turning hasNulls into a generic method: public static <T> boolean hasNulls(Pair<T> p) However, the version with the wildcard type seems easier to read. From the Library of Hristo Dimov Hristov 8.8 Wildcard Types 465 8.8.4 Wildcard Capture Let us write a method that swaps the elements of a pair: public static void swap(Pair<?> p) A wildcard is not a type variable, so we can’t write code that uses ? as a type. In other words, the following would be illegal: ? t = p.getFirst(); // ERROR p.setFirst(p.getSecond()); p.setSecond(t); That’s a problem because we need to temporarily hold the first element when we do the swapping. Fortunately, there is an interesting solution to this problem. We can write a helper method, swapHelper, like this: public static <T> void swapHelper(Pair<T> p) { T t = p.getFirst(); p.setFirst(p.getSecond()); p.setSecond(t); } Note that swapHelper is a generic method, whereas swap is not—it has a fixed parameter of type Pair<?>. Now we can call swapHelper from swap: public static void swap(Pair<?> p) { swapHelper(p); } In this case, the parameter T of the swapHelper method captures the wildcard. It isn’t known what type the wildcard denotes, but it is a definite type, and the definition of <T>swapHelper makes perfect sense when T denotes that type. Of course, in this case, we were not compelled to use a wildcard. We could have directly implemented <T> void swap(Pair<T> p) as a generic method without wildcards. However, consider this example in which a wildcard type occurs naturally in the middle of a computation: public static void maxminBonus(Manager[] a, Pair<? super Manager> result) { minmaxBonus(a, result); PairAlg.swapHelper(result); // OK--swapHelper captures wildcard type } Here, the wildcard capture mechanism cannot be avoided. Wildcard capture is only legal in very limited circumstances. The compiler must be able to guarantee that the wildcard represents a single, definite type. For example, the T in ArrayList<Pair<T>> can never capture the wildcard in From the Library of Hristo Dimov Hristov 466 Chapter 8 Generic Programming ArrayList<Pair<?>>. The array list might hold two Pair<?>, each of which has a different type for ?. The test program in Listing 8.3 gathers up the various methods that we discussed in the preceding sections so you can see them in context. Listing 8.3 pair3/PairTest3.java 1 package pair3; 2 3 4 5 6 7 8 9 10 11 12 13 14 /** * @version 1.01 2012-01-26 * @author Cay Horstmann */ public class PairTest3 { public static void main(String[] args) { var ceo = new Manager("Gus Greedy", 800000, 2003, 12, 15); var cfo = new Manager("Sid Sneaky", 600000, 2003, 12, 15); var buddies = new Pair<Manager>(ceo, cfo); printBuddies(buddies); 15 ceo.setBonus(1000000); cfo.setBonus(500000); Manager[] managers = { ceo, cfo }; 16 17 18 19 var result = new Pair<Employee>(); minmaxBonus(managers, result); System.out.println("first: " + result.getFirst().getName() + ", second: " + result.getSecond().getName()); maxminBonus(managers, result); System.out.println("first: " + result.getFirst().getName() + ", second: " + result.getSecond().getName()); 20 21 22 23 24 25 26 27 } 28 29 30 31 32 33 34 public static void printBuddies(Pair<? extends Employee> p) { Employee first = p.getFirst(); Employee second = p.getSecond(); System.out.println(first.getName() + " and " + second.getName() + " are buddies."); } 35 36 37 38 39 40 public static void minmaxBonus(Manager[] a, Pair<? super Manager> result) { if (a.length == 0) return; Manager min = a[0]; Manager max = a[0]; From the Library of Hristo Dimov Hristov 8.9 Reflection and Generics 467 for (int i = 1; i < a.length; i++) { if (min.getBonus() > a[i].getBonus()) min = a[i]; if (max.getBonus() < a[i].getBonus()) max = a[i]; } result.setFirst(min); result.setSecond(max); 41 42 43 44 45 46 47 } 48 49 public static void maxminBonus(Manager[] a, Pair<? super Manager> result) { minmaxBonus(a, result); PairAlg.swapHelper(result); // OK--swapHelper captures wildcard type } // can't write public static <T super manager> . . . 50 51 52 53 54 55 56 } 57 58 59 60 61 62 63 class PairAlg { public static boolean hasNulls(Pair<?> p) { return p.getFirst() == null || p.getSecond() == null; } 64 public static void swap(Pair<?> p) { swapHelper(p); } 65 66 public static <T> void swapHelper(Pair<T> p) { T t = p.getFirst(); p.setFirst(p.getSecond()); p.setSecond(t); } 67 68 69 70 71 72 73 } 8.9 Reflection and Generics Reflection lets you analyze arbitrary objects at runtime. If the objects are instances of generic classes, you don’t get much information about the generic type parameters because they have been erased. In the following sections, you will learn what you can nevertheless find out about generic classes with reflection. 8.9.1 The Generic Class Class The Class class is now generic. For example, String.class is actually an object (in fact, the sole object) of the class Class<String>. From the Library of Hristo Dimov Hristov 468 Chapter 8 Generic Programming The type parameter is useful because it allows the methods of Class<T> to be more specific about their return types. The following methods of Class<T> take advantage of the type parameter: T newInstance() T cast(Object obj) T[] getEnumConstants() Class<? super T> getSuperclass() Constructor<T> getConstructor(Class... parameterTypes) Constructor<T> getDeclaredConstructor(Class... parameterTypes) The newInstance method returns an instance of the class, obtained from the noargument constructor. Its return type can now be declared to be T, the same type as the class that is being described by Class<T>. That saves a cast. The cast method returns the given object, now declared as type T if its type is indeed a subtype of T. Otherwise, it throws a BadCastException. The getEnumConstants method returns null if this class is not an enum class or an array of the enumeration values which are known to be of type T. Finally, the getConstructor and getDeclaredConstructor methods return a Constructor<T> object. The Constructor class has also been made generic so that its newInstance method has the correct return type. java.lang.Class<T> 1.0 • T newInstance() returns a new instance constructed with the no-argument constructor. • T cast(Object obj) returns obj if it is null or can be converted to the type T, or throws a BadCastException otherwise. • T[] getEnumConstants() 5 returns an array of all values if T is an enumerated type, null otherwise. • Class<? super T> getSuperclass() returns the superclass of this class, or null if T is not a class or the class Object. • Constructor<T> getConstructor(Class... parameterTypes) 1.1 • Constructor<T> getDeclaredConstructor(Class... parameterTypes) 1.1 gets the public constructor, or the constructor with the given parameter types. From the Library of Hristo Dimov Hristov 8.9 Reflection and Generics 469 java.lang.reflect.Constructor<T> 1.1 • T newInstance(Object... parameters) returns a new instance constructed with the given parameters. 8.9.2 Using Class<T> Parameters for Type Matching It is sometimes useful to match the type variable of a Class<T> parameter in a generic method. Here is the canonical example: public static <T> Pair<T> makePair(Class<T> c) throws InstantiationException, IllegalAccessException { return new Pair<>(c.newInstance(), c.newInstance()); } If you call makePair(Employee.class) then Employee.class is an object of type Class<Employee>. The type parameter T of the makePair method matches Employee, and the compiler can infer that the method returns a Pair<Employee>. 8.9.3 Generic Type Information in the Virtual Machine One of the notable features of Java generics is the erasure of generic types in the virtual machine. Perhaps surprisingly, the erased classes still retain some faint memory of their generic origin. For example, the raw Pair class knows that it originated from the generic class Pair<T>, even though an object of type Pair can’t tell whether it was constructed as a Pair<String> or Pair<Employee>. Similarly, consider a method public static Comparable min(Comparable[] a) that is the erasure of a generic method public static <T extends Comparable<? super T>> T min(T[] a) You can use the reflection API to determine that • • • • • The The The The The generic method has a type parameter called T; type parameter has a subtype bound that is itself a generic type; bounding type has a wildcard parameter; wildcard parameter has a supertype bound; and generic method has a generic array parameter. From the Library of Hristo Dimov Hristov 470 Chapter 8 Generic Programming In other words, you can reconstruct everything about generic classes and methods that their implementors declared. However, you won’t know how the type parameters were resolved for specific objects or method calls. In order to express generic type declarations, use the interface Type in the java.lang.reflect package. The interface has the following subtypes: • The Class class, describing concrete types • The TypeVariable interface, describing type variables (such as T extends Comparable<? super T>) • The WildcardType interface, describing wildcards (such as ? super T) • The ParameterizedType interface, describing generic class or interface types (such as Comparable<? super T>) • The GenericArrayType interface, describing generic arrays (such as T[]) Figure 8.5 shows the inheritance hierarchy. Note that the last four subtypes are interfaces—the virtual machine instantiates suitable classes that implement these interfaces. Figure 8.5 The Type interface and its descendants Listing 8.4 uses the generic reflection API to print out what it discovers about a given class. If you run it with the Pair class, you get this report: class Pair<T> extends java.lang.Object public T getFirst() public T getSecond() public void setFirst(T) public void setSecond(T) From the Library of Hristo Dimov Hristov 8.9 Reflection and Generics 471 If you run it with ArrayAlg in the PairTest2 directory, the report displays the following method: public static <T extends java.lang.Comparable> Pair<T> minmax(T[]) Listing 8.4 genericReflection/GenericReflectionTest.java 1 package genericReflection; 2 3 4 import java.lang.reflect.*; import java.util.*; 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 /** * @version 1.11 2018-04-10 * @author Cay Horstmann */ public class GenericReflectionTest { public static void main(String[] args) { // read class name from command line args or user input String name; if (args.length > 0) name = args[0]; else { try (var in = new Scanner(System.in)) { System.out.println("Enter class name (e.g., java.util.Collections): "); name = in.next(); } } 25 try { 26 27 // print generic info for class and public methods Class<?> cl = Class.forName(name); printClass(cl); for (Method m : cl.getDeclaredMethods()) printMethod(m); 28 29 30 31 32 } catch (ClassNotFoundException e) { e.printStackTrace(); } 33 34 35 36 37 38 } 39 40 41 public static void printClass(Class<?> cl) { (Continues) From the Library of Hristo Dimov Hristov 472 Chapter 8 Generic Programming Listing 8.4 (Continued) System.out.print(cl); printTypes(cl.getTypeParameters(), "<", ", ", ">", true); Type sc = cl.getGenericSuperclass(); if (sc != null) { System.out.print(" extends "); printType(sc, false); } printTypes(cl.getGenericInterfaces(), " implements ", ", ", "", false); System.out.println(); 42 43 44 45 46 47 48 49 50 51 52 } 53 54 55 56 57 58 59 public static void printMethod(Method m) { String name = m.getName(); System.out.print(Modifier.toString(m.getModifiers())); System.out.print(" "); printTypes(m.getTypeParameters(), "<", ", ", "> ", true); 60 printType(m.getGenericReturnType(), false); System.out.print(" "); System.out.print(name); System.out.print("("); printTypes(m.getGenericParameterTypes(), "", ", ", "", false); System.out.println(")"); 61 62 63 64 65 66 67 } 68 69 70 71 72 73 74 75 76 77 78 79 80 81 public static void printTypes(Type[] types, String pre, String sep, String suf, boolean isDefinition) { if (pre.equals(" extends ") && Arrays.equals(types, new Type[] { Object.class })) return; if (types.length > 0) System.out.print(pre); for (int i = 0; i < types.length; i++) { if (i > 0) System.out.print(sep); printType(types[i], isDefinition); } if (types.length > 0) System.out.print(suf); } 82 83 84 85 86 public static void printType(Type type, boolean isDefinition) { if (type instanceof Class) { From the Library of Hristo Dimov Hristov 8.9 473 var t = (Class<?>) type; System.out.print(t.getName()); 87 88 } else if (type instanceof TypeVariable) { var t = (TypeVariable<?>) type; System.out.print(t.getName()); if (isDefinition) printTypes(t.getBounds(), " extends ", " & ", "", false); } else if (type instanceof WildcardType) { var t = (WildcardType) type; System.out.print("?"); printTypes(t.getUpperBounds(), " extends ", " & ", "", false); printTypes(t.getLowerBounds(), " super ", " & ", "", false); } else if (type instanceof ParameterizedType) { var t = (ParameterizedType) type; Type owner = t.getOwnerType(); if (owner != null) { printType(owner, false); System.out.print("."); } printType(t.getRawType(), false); printTypes(t.getActualTypeArguments(), "<", ", ", ">", false); } else if (type instanceof GenericArrayType) { var t = (GenericArrayType) type; System.out.print(""); printType(t.getGenericComponentType(), isDefinition); System.out.print("[]"); } 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 } 123 124 Reflection and Generics } 8.9.4 Type Literals Sometimes, you want to drive program behavior by the type of a value. For example, in a persistence mechanism, you may want the user to specify a way of saving an object of a particular class. This is typically implemented by associating the Class object with an action. However, with generic classes, erasure poses a problem. How can you have different actions for, say, ArrayList<Integer> and ArrayList<String> when both erase to the same raw ArrayList type? From the Library of Hristo Dimov Hristov 474 Chapter 8 Generic Programming There is a trick that can offer relief in some situations. You can capture an instance of the Type interface that you encountered in the preceding section. Construct an anonymous subclass like this: var type = new TypeLiteral<ArrayList<Integer>>(){} // note the {} The TypeLiteral constructor captures the generic supertype: class TypeLiteral { public TypeLiteral() { Type parentType = getClass().getGenericSuperclass(); if (parentType instanceof ParameterizedType) { type = ((ParameterizedType) parentType).getActualTypeArguments()[0]; } else throw new UnsupportedOperationException( "Construct as new TypeLiteral<. . .>(){}"); } . . . } If we have a generic type available at runtime, we can match it against the TypeLiteral. We can’t get a generic type from an object—it is erased. But, as you have seen in the preceding section, generic types of fields and method parameters survive in the virtual machine. Injection frameworks such as CDI and Guice use type literals to control injection of generic types. The example program in the book’s companion code shows a simpler example. Given an object, we enumerate its fields, whose generic types are available, and look up associated formatting actions. We format an ArrayList<Integer> by separating the values with spaces, an by joining the characters to a string. Any other array lists are formatted by ArrayList.toString. ArrayList<Character> Listing 8.5 genericReflection/TypeLiterals.java 1 package genericReflection; 2 3 /** @version 1.01 2018-04-10 @author Cay Horstmann 4 5 6 */ 7 8 9 import java.lang.reflect.*; import java.util.*; From the Library of Hristo Dimov Hristov 8.9 10 Reflection and Generics 475 import java.util.function.*; 11 12 13 14 15 16 17 /** * A type literal describes a type that can be generic, such as ArrayList<String>. */ class TypeLiteral<T> { private Type type; 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 /** * This constructor must be invoked from an anonymous subclass * as new TypeLiteral<. . .>(){} */ public TypeLiteral() { Type parentType = getClass().getGenericSuperclass(); if (parentType instanceof ParameterizedType) { type = ((ParameterizedType) parentType).getActualTypeArguments()[0]; } else throw new UnsupportedOperationException( "Construct as new TypeLiteral<. . .>(){}"); } 34 35 36 37 38 private TypeLiteral(Type type) { this.type = type; } 39 40 41 42 43 44 45 46 /** * Yields a type literal that describes the given type. */ public static TypeLiteral<?> of(Type type) { return new TypeLiteral<Object>(type); } 47 48 49 50 51 52 public String toString() { if (type instanceof Class) return ((Class<?>) type).getName(); else return type.toString(); } 53 54 55 56 57 58 public boolean equals(Object otherObject) { return otherObject instanceof TypeLiteral && type.equals(((TypeLiteral<?>) otherObject).type); } (Continues) From the Library of Hristo Dimov Hristov 476 Chapter 8 Generic Programming Listing 8.5 (Continued) 59 public int hashCode() { return type.hashCode(); } 60 61 62 63 64 } 65 66 67 68 69 70 71 /** * Formats objects, using rules that associate types with formatting functions. */ class Formatter { private Map<TypeLiteral<?>, Function<?, String>> rules = new HashMap<>(); 72 73 74 75 76 77 78 79 80 81 /** * Add a formatting rule to this formatter. * @param type the type to which this rule applies * @param formatterForType the function that formats objects of this type */ public <T> void forType(TypeLiteral<T> type, Function<T, String> formatterForType) { rules.put(type, formatterForType); } 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 /** * Formats all fields of an object using the rules of this formatter. * @param obj an object * @return a string with all field names and formatted values */ public String formatFields(Object obj) throws IllegalArgumentException, IllegalAccessException { var result = new StringBuilder(); for (Field f : obj.getClass().getDeclaredFields()) { result.append(f.getName()); result.append("="); f.setAccessible(true); Function<?, String> formatterForType = rules.get(TypeLiteral.of(f.getGenericType())); if (formatterForType != null) { // formatterForType has parameter type ?. Nothing can be passed to its apply // method. Cast makes the parameter type to Object so we can invoke it. @SuppressWarnings("unchecked") Function<Object, String> objectFormatter = (Function<Object, String>) formatterForType; From the Library of Hristo Dimov Hristov 8.9 477 result.append(objectFormatter.apply(f.get(obj))); } else result.append(f.get(obj).toString()); result.append("\n"); 105 106 107 108 109 } return result.toString(); 110 111 } 112 113 Reflection and Generics } 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 public class TypeLiterals { public static class Sample { ArrayList<Integer> nums; ArrayList<Character> chars; ArrayList<String> strings; public Sample() { nums = new ArrayList<>(); nums.add(42); nums.add(1729); chars = new ArrayList<>(); chars.add('H'); chars.add('i'); strings = new ArrayList<>(); strings.add("Hello"); strings.add("World"); } } 132 private static <T> String join(String separator, ArrayList<T> elements) { var result = new StringBuilder(); for (T e : elements) { if (result.length() > 0) result.append(separator); result.append(e.toString()); } return result.toString(); } 133 134 135 136 137 138 139 140 141 142 143 public static void main(String[] args) throws Exception { var formatter = new Formatter(); formatter.forType(new TypeLiteral<ArrayList<Integer>>(){}, lst -> join(" ", lst)); formatter.forType(new TypeLiteral<ArrayList<Character>>(){}, lst -> "\"" + join("", lst) + "\""); System.out.println(formatter.formatFields(new Sample())); } 144 145 146 147 148 149 150 151 152 153 } From the Library of Hristo Dimov Hristov 478 Chapter 8 Generic Programming java.lang.Class<T> 1.0 • TypeVariable[] getTypeParameters() 5 gets the generic type variables if this type was declared as a generic type, or an array of length 0 otherwise. • Type getGenericSuperclass() 5 gets the generic type of the superclass that was declared for this type, or null if this type is Object or not a class type. • Type[] getGenericInterfaces() 5 gets the generic types of the interfaces that were declared for this type, in declaration order, or an array of length 0 if this type doesn’t implement interfaces. java.lang.reflect.Method 1.1 • TypeVariable[] getTypeParameters() 5 gets the generic type variables if this method was declared as a generic method, or an array of length 0 otherwise. • Type getGenericReturnType() 5 gets the generic return type with which this method was declared. • Type[] getGenericParameterTypes() 5 gets the generic parameter types with which this method was declared. If the method has no parameters, an array of length 0 is returned. java.lang.reflect.TypeVariable 5 • String getName() gets the name of this type variable. • Type[] getBounds() gets the subclass bounds of this type variable, or an array of length 0 if the variable is unbounded. From the Library of Hristo Dimov Hristov 8.9 Reflection and Generics 479 java.lang.reflect.WildcardType 5 • Type[] getUpperBounds() gets the subclass (extends) bounds of this type variable, or an array of length 0 if the variable has no subclass bounds. • Type[] getLowerBounds() gets the superclass (super) bounds of this type variable, or an array of length 0 if the variable has no superclass bounds. java.lang.reflect.ParameterizedType 5 • Type getRawType() gets the raw type of this parameterized type. • Type[] getActualTypeArguments() gets the type parameters with which this parameterized type was declared. • Type getOwnerType() gets the outer class type if this is an inner type, or null if this is a top-level type. java.lang.reflect.GenericArrayType 5 • Type getGenericComponentType() gets the generic component type with which this array type was declared. You now know how to use generic classes and how to program your own generic classes and methods if the need arises. Just as importantly, you know how to decipher the generic type declarations that you may encounter in the API documentation and in error messages. For an exhaustive discussion of everything there is to know about Java generics, turn to Angelika Langer’s excellent list of frequently (and not so frequently) asked questions at http://angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html. In the next chapter, you will see how the Java collections framework puts generics to work. From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov CHAPTER 9 Collections In this chapter • 9.1 The Java Collections Framework, page 482 • 9.2 Interfaces in the Collections Framework, page 492 • 9.3 Concrete Collections, page 494 • 9.4 Maps, page 519 • 9.5 Views and Wrappers, page 532 • 9.6 Algorithms, page 541 • 9.7 Legacy Collections, page 552 The data structures that you choose can make a big difference when you try to implement methods in a natural style or are concerned with performance. Do you need to search quickly through thousands (or even millions) of sorted items? Do you need to rapidly insert and remove elements in the middle of an ordered sequence? Do you need to establish associations between keys and values? This chapter shows how the Java library can help you accomplish the traditional data structuring needed for serious programming. In college computer science programs, a course called Data Structures usually takes a semester to complete, and there are many, many books devoted to this important topic. Our coverage differs from that of a college course; we will skip the theory and just show you how to use the collection classes in the standard library. 481 From the Library of Hristo Dimov Hristov 482 Chapter 9 Collections 9.1 The Java Collections Framework The initial release of Java supplied only a small set of classes for the most useful data structures: Vector, Stack, Hashtable, BitSet, and the Enumeration interface that provides an abstract mechanism for visiting elements in an arbitrary container. That was certainly a wise choice—it takes time and skill to come up with a comprehensive collection class library. With the advent of Java 1.2, the designers felt that the time had come to roll out a full-fledged set of data structures. They faced a number of conflicting design challenges. They wanted the library to be small and easy to learn. They did not want the complexity of the Standard Template Library (or STL) of C++, but they wanted the benefit of “generic algorithms” that STL pioneered. They wanted the legacy classes to fit into the new framework. As all designers of collections libraries do, they had to make some hard choices, and they came up with a number of idiosyncratic design decisions along the way. In this section, we will explore the basic design of the Java collections framework, show you how to put it to work, and explain the reasoning behind some of the more controversial features. 9.1.1 Separating Collection Interfaces and Implementation As is common with modern data structure libraries, the Java collection library separates interfaces and implementations. Let us look at that separation with a familiar data structure, the queue. A queue interface specifies that you can add elements at the tail end of the queue, remove them at the head, and find out how many elements are in the queue. You use a queue when you need to collect objects and retrieve them in a “first in, first out” fashion (see Figure 9.1). Figure 9.1 A queue From the Library of Hristo Dimov Hristov 9.1 The Java Collections Framework 483 A minimal form of a queue interface might look like this: public interface Queue<E> // a simplified form of the interface in the standard library { void add(E element); E remove(); int size(); } The interface tells you nothing about how the queue is implemented. Of the two common implementations of a queue, one uses a “circular array” and one uses a linked list (see Figure 9.2). Figure 9.2 Queue implementations Each implementation can be expressed by a class that implements the Queue interface. public class CircularArrayQueue<E> implements Queue<E> // not an actual library class { private int head; private int tail; From the Library of Hristo Dimov Hristov 484 Chapter 9 Collections CircularArrayQueue(int capacity) { . . . } public void add(E element) { . . . } public E remove() { . . . } public int size() { . . . } private E[] elements; } public class LinkedListQueue<E> implements Queue<E> // not an actual library class { private Link head; private Link tail; LinkedListQueue() public void add(E public E remove() public int size() { . . . } element) { . . . } { . . . } { . . . } } NOTE: The Java library doesn’t actually have classes named CircularArrayQueue and LinkedListQueue. We use these classes as examples to explain the conceptual distinction between collection interfaces and implementations. If you need a circular array queue, use the ArrayDeque class. For a linked list queue, simply use the LinkedList class—it implements the Queue interface. When you use a queue in your program, you don’t need to know which implementation is actually used once the collection has been constructed. Therefore, it makes sense to use the concrete class only when you construct the collection object. Use the interface type to hold the collection reference. Queue<Customer> expressLane = new CircularArrayQueue<>(100); expressLane.add(new Customer("Harry")); With this approach, if you change your mind, you can easily use a different implementation. You only need to change your program in one place—in the constructor call. If you decide that a LinkedListQueue is a better choice after all, your code becomes Queue<Customer> expressLane = new LinkedListQueue<>(); expressLane.add(new Customer("Harry")); Why would you choose one implementation over another? The interface says nothing about the efficiency of an implementation. A circular array is somewhat more efficient than a linked list, so it is generally preferable. However, as usual, there is a price to pay. From the Library of Hristo Dimov Hristov 9.1 The Java Collections Framework 485 The circular array is a bounded collection—it has a finite capacity. If you don’t have an upper limit on the number of objects that your program will collect, you may be better off with a linked list implementation after all. When you study the API documentation, you will find another set of classes whose name begins with Abstract, such as AbstractQueue. These classes are intended for library implementors. In the (perhaps unlikely) event that you want to implement your own queue class, you will find it easier to extend AbstractQueue than to implement all the methods of the Queue interface. 9.1.2 The Collection Interface The fundamental interface for collection classes in the Java library is the Collection interface. The interface has two fundamental methods: public interface Collection<E> { boolean add(E element); Iterator<E> iterator(); . . . } There are several methods in addition to these two; we will discuss them later. The add method adds an element to the collection. The add method returns true if adding the element actually changes the collection, and false if the collection is unchanged. For example, if you try to add an object to a set and the object is already present, the add request has no effect because sets reject duplicates. The iterator method returns an object that implements the Iterator interface. You can use the iterator object to visit the elements in the collection one by one. We discuss iterators in the next section. 9.1.3 Iterators The Iterator interface has four methods: public interface Iterator<E> { E next(); boolean hasNext(); void remove(); default void forEachRemaining(Consumer<? super E> action); } From the Library of Hristo Dimov Hristov 486 Chapter 9 Collections By repeatedly calling the next method, you can visit the elements from the collection one by one. However, if you reach the end of the collection, the next method throws a NoSuchElementException. Therefore, you need to call the hasNext method before calling next. That method returns true if the iterator object still has more elements to visit. If you want to inspect all elements in a collection, request an iterator and then keep calling the next method while hasNext returns true. For example: Collection<String> c = . . .; Iterator<String> iter = c.iterator(); while (iter.hasNext()) { String element = iter.next(); do something with element } You can write such a loop more concisely as the “for each” loop: for (String element : c) { do something with element } The compiler simply translates the “for each” loop into a loop with an iterator. The “for each” loop works with any object that implements the Iterable interface, an interface with a single abstract method: public interface Iterable<E> { Iterator<E> iterator(); . . . } The Collection interface extends the Iterable interface. Therefore, you can use the “for each” loop with any collection in the standard library. Instead of writing a loop, you can call the forEachRemaining method with a lambda expression that consumes an element. The lambda expression is invoked with each element of the iterator, until there are none left. iterator.forEachRemaining(element -> do something with element); The order in which the elements are visited depends on the collection type. If you iterate over an ArrayList, the iterator starts at index 0 and increments the index in each step. However, if you visit the elements in a HashSet, you will get them in an essentially random order. You can be assured that you will encounter all elements of the collection during the course of the iteration, From the Library of Hristo Dimov Hristov 9.1 The Java Collections Framework 487 but you cannot make any assumptions about their ordering. This is usually not a problem because the ordering does not matter for computations such as computing totals or counting matches. NOTE: Old-timers will notice that the next and hasNext methods of the Iterator interface serve the same purpose as the nextElement and hasMoreElements methods of an Enumeration. The designers of the Java collections library could have chosen to make use of the Enumeration interface. But they disliked the cumbersome method names and instead introduced a new interface with shorter method names. There is an important conceptual difference between iterators in the Java collections library and iterators in other libraries. In traditional collections libraries, such as the Standard Template Library of C++, iterators are modeled after array indexes. Given such an iterator, you can look up the element that is stored at that position, much like you can look up an array element a[i] if you have an array index i. Independently of the lookup, you can advance the iterator to the next position. This is the same operation as advancing an array index by calling i++, without performing a lookup. However, the Java iterators do not work like that. The lookup and position change are tightly coupled. The only way to look up an element is to call next, and that lookup advances the position. Instead, think of Java iterators as being between elements. When you call next, the iterator jumps over the next element, and it returns a reference to the element that it just passed (see Figure 9.3). NOTE: Here is another useful analogy. You can think of Iterator.next as the equivalent of InputStream.read. Reading a byte from a stream automatically “consumes” the byte. The next call to read consumes and returns the next byte from the input. Similarly, repeated calls to next let you read all elements in a collection. The remove method of the Iterator interface removes the element that was returned by the last call to next. In many situations, that makes sense—you need to see the element before you can decide that it is the one that should be removed. But if you want to remove an element in a particular position, you still need to skip past the element. For example, here is how you remove the first element in a collection of strings: From the Library of Hristo Dimov Hristov 488 Chapter 9 Collections Figure 9.3 Advancing an iterator Iterator<String> it = c.iterator(); it.next(); // skip over the first element it.remove(); // now remove it More importantly, there is a dependency between the calls to the next and remove methods. It is illegal to call remove if it wasn’t preceded by a call to next. If you try, an IllegalStateException is thrown. If you want to remove two adjacent elements, you cannot simply call it.remove(); it.remove(); // ERROR Instead, you must first call next to jump over the element to be removed. it.remove(); it.next(); it.remove(); // OK From the Library of Hristo Dimov Hristov 9.1 The Java Collections Framework 489 9.1.4 Generic Utility Methods The Collection and Iterator interfaces are generic, which means you can write utility methods that operate on any kind of collection. For example, here is a generic method that tests whether an arbitrary collection contains a given element: public static <E> boolean contains(Collection<E> c, Object obj) { for (E element : c) if (element.equals(obj)) return true; return false; } The designers of the Java library decided that some of these utility methods are so useful that the library should make them available. That way, library users don’t have to keep reinventing the wheel. The contains method is one such method. In fact, the Collection interface declares quite a few useful methods that all implementing classes must supply. Among them are int size() boolean isEmpty() boolean contains(Object obj) boolean containsAll(Collection<?> c) boolean equals(Object other) boolean addAll(Collection<? extends E> from) boolean remove(Object obj) boolean removeAll(Collection<?> c) void clear() boolean retainAll(Collection<?> c) Object[] toArray() <T> T[] toArray(T[] arrayToFill) Many of these methods are self-explanatory; you will find full documentation in the API notes at the end of this section. Of course, it is a bother if every class that implements the Collection interface has to supply so many routine methods. To make life easier for implementors, the library supplies a class AbstractCollection that leaves the fundamental methods size and iterator abstract but implements the routine methods in terms of them. For example: From the Library of Hristo Dimov Hristov 490 Chapter 9 Collections public abstract class AbstractCollection<E> implements Collection<E> { . . . public abstract Iterator<E> iterator(); public boolean contains(Object obj) { for (E element : this) // calls iterator() if (element.equals(obj)) return true; return false; } . . . } A concrete collection class can now extend the AbstractCollection class. It is up to the concrete collection class to supply an iterator method, but the contains method has been taken care of by the AbstractCollection superclass. However, if the subclass has a more efficient way of implementing contains, it is free to do so. This approach is a bit outdated. It would be nicer if the methods were default methods of the Collection interface. This has not happened. However, several default methods have been added. Most of them deal with streams (which we will discuss in Volume II). In addition, there is a useful method default boolean removeIf(Predicate<? super E> filter) for removing elements that fulfill a condition. java.util.Collection<E> 1.2 • Iterator<E> iterator() returns an iterator that can be used to visit the elements in the collection. • int size() returns the number of elements currently stored in the collection. • boolean isEmpty() returns true if this collection contains no elements. • boolean contains(Object obj) returns true if this collection contains an object equal to obj. (Continues) From the Library of Hristo Dimov Hristov 9.1 The Java Collections Framework 491 java.util.Collection<E> 1.2 (Continued) • boolean containsAll(Collection<?> other) returns true if this collection contains all elements in the other collection. • boolean add(E element) adds an element to the collection. Returns true if the collection changed as a result of this call. • boolean addAll(Collection<? extends E> other) adds all elements from the other collection to this collection. Returns true if the collection changed as a result of this call. • boolean remove(Object obj) removes an object equal to obj from this collection. Returns true if a matching object was removed. • boolean removeAll(Collection<?> other) removes from this collection all elements from the other collection. Returns true if the collection changed as a result of this call. • default boolean removeIf(Predicate<? super E> filter) 8 removes all elements for which filter returns true. Returns true if the collection changed as a result of this call. • void clear() removes all elements from this collection. • boolean retainAll(Collection<?> other) removes all elements from this collection that do not equal one of the elements in the other collection. Returns true if the collection changed as a result of this call. • Object[] toArray() returns an array of the objects in the collection. • <T> T[] toArray(T[] arrayToFill) returns an array of the objects in the collection. If arrayToFill has sufficient length, it is filled with the elements of this collection. If there is space, a null element is appended. Otherwise, a new array with the same component type as arrayToFill and the same length as the size of this collection is allocated and filled. From the Library of Hristo Dimov Hristov 492 Chapter 9 Collections java.util.Iterator<E> 1.2 • boolean hasNext() returns true if there is another element to visit. • E next() returns the next object to visit. Throws a NoSuchElementException if the end of the collection has been reached. • void remove() removes the last visited object. This method must immediately follow an element visit. If the collection has been modified since the last element visit, this method throws an IllegalStateException. • default void forEachRemaining(Consumer<? super E> action) 8 visits elements and passes them to the given action until no elements remain or the action throws an exception. 9.2 Interfaces in the Collections Framework The Java collections framework defines a number of interfaces for different types of collections, shown in Figure 9.4. There are two fundamental interfaces for collections: Collection and Map. As you already saw, you insert elements into a collection with a method boolean add(E element) However, maps hold key/value pairs, and you use the put method to insert them: V put(K key, V value) To read elements from a collection, visit them with an iterator. However, you can read values from a map with the get method: V get(K key) A List is an ordered collection. Elements are added into a particular position in the container. An element can be accessed in two ways: by an iterator or by an integer index. The latter is called random access because elements can be visited in any order. In contrast, when using an iterator, one must visit them sequentially. From the Library of Hristo Dimov Hristov 9.2 Interfaces in the Collections Framework 493 Figure 9.4 The interfaces of the collections framework The List interface defines several methods for random access: void add(int index, E element) void remove(int index) E get(int index) E set(int index, E element) The ListIterator interface is a subinterface of Iterator. It defines a method for adding an element before the iterator position: void add(E element) Frankly, this aspect of the collections framework is poorly designed. In practice, there are two kinds of ordered collections, with very different performance tradeoffs. An ordered collection that is backed by an array has fast random access, and it makes sense to use the List methods with an integer index. In From the Library of Hristo Dimov Hristov 494 Chapter 9 Collections contrast, a linked list, while also ordered, has slow random access, and it is best traversed with an iterator. It would have been an easy matter to provide two interfaces. NOTE: To avoid carrying out random access operations for linked lists, Java 1.4 introduced a tagging interface, RandomAccess. That interface has no methods, but you can use it to test whether a particular collection supports efficient random access: if (c instanceof RandomAccess) { use random access algorithm } else { use sequential access algorithm } The Set interface is identical to the Collection interface, but the behavior of the methods is more tightly defined. The add method of a set should reject duplicates. The equals method of a set should be defined so that two sets are identical if they have the same elements, but not necessarily in the same order. The hashCode method should be defined so that two sets with the same elements yield the same hash code. Why make a separate interface if the method signatures are the same? Conceptually, not all collections are sets. Making a Set interface enables programmers to write methods that accept only sets. The SortedSet and SortedMap interfaces expose the comparator object used for sorting, and they define methods to obtain views of subsets of the collections. We discuss these in Section 9.5, “Views and Wrappers,” on p. 532. Finally, Java 6 introduced interfaces NavigableSet and NavigableMap that contain additional methods for searching and traversal in sorted sets and maps. (Ideally, these methods should have simply been included in the SortedSet and SortedMap interface.) The TreeSet and TreeMap classes implement these interfaces. 9.3 Concrete Collections Table 9.1 shows the collections in the Java library and briefly describes the purpose of each collection class. (For simplicity, we omit the thread-safe From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 495 collections that will be discussed in Chapter 12.) All classes in Table 9.1 implement the Collection interface, with the exception of the classes with names ending in Map. Those classes implement the Map interface instead. We will discuss maps in Section 9.4, “Maps,” on p. 519. Figure 9.5 shows the relationships between these classes. Table 9.1 Concrete Collections in the Java Library Collection Type Description See Page ArrayList An indexed sequence that grows and shrinks dynamically 507 LinkedList An ordered sequence that allows efficient insertion and removal at any location 496 ArrayDeque A double-ended queue that is implemented as a circular array 516 HashSet An unordered collection that rejects duplicates 507 TreeSet A sorted set 511 EnumSet A set of enumerated type values 529 LinkedHashSet A set that remembers the order in which elements were inserted 527 PriorityQueue A collection that allows efficient removal of the smallest element 518 HashMap A data structure that stores key/value associations 526 TreeMap A map in which the keys are sorted 519 EnumMap A map in which the keys belong to an enumerated type 529 LinkedHashMap A map that remembers the order in which entries were added 527 WeakHashMap A map with values that can be reclaimed by the garbage collector if they are not used elsewhere 526 A map with keys that are compared by ==, not 530 IdentityHashMap equals From the Library of Hristo Dimov Hristov 496 Chapter 9 Collections Figure 9.5 Classes in the collections framework 9.3.1 Linked Lists We already used arrays and their dynamic cousin, the ArrayList class, for many examples in this book. However, arrays and array lists suffer from a major drawback. Removing an element from the middle of an array is expensive since all array elements beyond the removed one must be moved toward the beginning of the array (see Figure 9.6). The same is true for inserting elements in the middle. From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 497 Figure 9.6 Removing an element from an array Another well-known data structure, the linked list, solves this problem. Where an array stores object references in consecutive memory locations, a linked list stores each object in a separate link. Each link also stores a reference to the next link in the sequence. In the Java programming language, all linked lists are actually doubly linked; that is, each link also stores a reference to its predecessor (see Figure 9.7). Figure 9.7 A doubly linked list From the Library of Hristo Dimov Hristov 498 Chapter 9 Collections Removing an element from the middle of a linked list is an inexpensive operation—only the links around the element to be removed need to be updated (see Figure 9.8). Figure 9.8 Removing an element from a linked list Perhaps you once took a data structures course in which you learned how to implement linked lists. You may have bad memories of tangling up the links when removing or adding elements in the linked list. If so, you will be pleased to learn that the Java collections library supplies a class LinkedList ready for you to use. The following code example adds three elements and then removes the second one: var staff = new LinkedList<String>(); staff.add("Amy"); staff.add("Bob"); staff.add("Carl"); Iterator<String> iter = staff.iterator(); String first = iter.next(); // visit first element String second = iter.next(); // visit second element iter.remove(); // remove last visited element There is, however, an important difference between linked lists and generic collections. A linked list is an ordered collection in which the position of the objects matters. The LinkedList.add method adds the object to the end of the list. From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 499 But you will often want to add objects somewhere in the middle of a list. This position-dependent add method is the responsibility of an iterator, since iterators describe positions in collections. Using iterators to add elements makes sense only for collections that have a natural ordering. For example, the set data type that we discuss in the next section does not impose any ordering on its elements. Therefore, there is no add method in the Iterator interface. Instead, the collections library supplies a subinterface ListIterator that contains an add method: interface ListIterator<E> extends Iterator<E> { void add(E element); . . . } Unlike Collection.add, this method does not return a boolean—it is assumed that the add operation always modifies the list. In addition, the ListIterator interface has two methods that you can use for traversing a list backwards. E previous() boolean hasPrevious() Like the next method, the previous method returns the object that it skipped over. The listIterator method of the LinkedList class returns an iterator object that implements the ListIterator interface. ListIterator<String> iter = staff.listIterator(); The add method adds the new element before the iterator position. For example, the following code skips past the first element in the linked list and adds "Juliet" before the second element (see Figure 9.9): var staff = new LinkedList<String>(); staff.add("Amy"); staff.add("Bob"); staff.add("Carl"); ListIterator<String> iter = staff.listIterator(); iter.next(); // skip past first element iter.add("Juliet"); If you call the add method multiple times, the elements are simply added in the order in which you supplied them. They are all added in turn before the current iterator position. When you use the add operation with an iterator that was freshly returned from the listIterator method and that points to the beginning of the linked list, the newly added element becomes the new head of the list. When the From the Library of Hristo Dimov Hristov 500 Chapter 9 Collections Figure 9.9 Adding an element to a linked list iterator has passed the last element of the list (that is, when hasNext returns false), the added element becomes the new tail of the list. If the linked list has n elements, there are n + 1 spots for adding a new element. These spots correspond to the n + 1 possible positions of the iterator. For example, if a linked list contains three elements, A, B, and C, there are four possible positions (marked as |) for inserting a new element: |ABC A|BC AB|C ABC| From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 501 NOTE: Be careful with the “cursor” analogy. The remove operation does not work exactly like the Backspace key. Immediately after a call to next, the remove method indeed removes the element to the left of the iterator, just like the Backspace key would. However, if you have just called previous, the element to the right will be removed. And you can’t call remove twice in a row. Unlike the add method, which depends only on the iterator position, the remove method depends on the iterator state. Finally, a set method replaces the last element, returned by a call to next or previous, with a new element. For example, the following code replaces the first element of a list with a new value: ListIterator<String> iter = list.listIterator(); String oldValue = iter.next(); // returns first element iter.set(newValue); // sets first element to newValue As you might imagine, if an iterator traverses a collection while another iterator is modifying it, confusing situations can occur. For example, suppose an iterator points before an element that another iterator has just removed. The iterator is now invalid and should no longer be used. The linked list iterators have been designed to detect such modifications. If an iterator finds that its collection has been modified by another iterator or by a method of the collection itself, it throws a ConcurrentModificationException. For example, consider the following code: List<String> list = . . .; ListIterator<String> iter1 = list.listIterator(); ListIterator<String> iter2 = list.listIterator(); iter1.next(); iter1.remove(); iter2.next(); // throws ConcurrentModificationException The call to iter2.next throws a ConcurrentModificationException since iter2 detects that the list was modified externally. To avoid concurrent modification exceptions, follow this simple rule: You can attach as many iterators to a collection as you like, provided that all of them are only readers. Alternatively, you can attach a single iterator that can both read and write. Concurrent modification detection is done in a simple way. The collection keeps track of the number of mutating operations (such as adding and removing elements). Each iterator keeps a separate count of the number of mutating operations that it was responsible for. At the beginning of each iterator From the Library of Hristo Dimov Hristov 502 Chapter 9 Collections method, the iterator simply checks whether its own mutation count equals that of the collection. If not, it throws a ConcurrentModificationException. NOTE: There is, however, a curious exception to the detection of concurrent modifications. The linked list only keeps track of structural modifications to the list, such as adding and removing links. The set method does not count as a structural modification. You can attach multiple iterators to a linked list, all of which call set to change the contents of existing links. This capability is required for a number of algorithms in the Collections class that we discuss later in this chapter. Now you have seen the fundamental methods of the LinkedList class. Use a ListIterator to traverse the elements of the linked list in either direction and to add and remove elements. As you saw in Section 9.2, “Interfaces in the Collections Framework,” on p. 492, many other useful methods for operating on linked lists are declared in the Collection interface. These are, for the most part, implemented in the AbstractCollection superclass of the LinkedList class. For example, the toString method invokes toString on all elements and produces one long string of the format [A, B, C]. This is handy for debugging. Use the contains method to check whether an element is present in a linked list. For example, the call staff.contains("Harry") returns true if the linked list already contains a string equal to the string "Harry". The library also supplies a number of methods that are, from a theoretical perspective, somewhat dubious. Linked lists do not support fast random access. If you want to see the nth element of a linked list, you have to start at the beginning and skip past the first n – 1 elements. There is no shortcut. For that reason, programmers don’t usually use linked lists in situations where elements need to be accessed by an integer index. Nevertheless, the LinkedList class supplies a get method that lets you access a particular element: LinkedList<String> list = . . .; String obj = list.get(n); Of course, this method is not very efficient. If you find yourself using it, you are probably using a wrong data structure for your problem. You should never use this illusory random access method to step through a linked list. The code for (int i = 0; i < list.size(); i++) do something with list.get(i); From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 503 is staggeringly inefficient. Each time you look up another element, the search starts again from the beginning of the list. The LinkedList object makes no effort to cache the position information. NOTE: The get method has one slight optimization: If the index is at least size() / 2, the search for the element starts at the end of the list. The list iterator interface also has a method to tell you the index of the current position. In fact, since Java iterators conceptually point between elements, it has two of them: The nextIndex method returns the integer index of the element that would be returned by the next call to next; the previousIndex method returns the index of the element that would be returned by the next call to previous. Of course, that is simply one less than nextIndex. These methods are efficient—an iterator keeps a count of its current position. Finally, if you have an integer index n, then list.listIterator(n) returns an iterator that points just before the element with index n. That is, calling next yields the same element as list.get(n); obtaining that iterator is inefficient. If you have a linked list with only a handful of elements, you don’t have to be overly paranoid about the cost of the get and set methods. But then, why use a linked list in the first place? The only reason to use a linked list is to minimize the cost of insertion and removal in the middle of the list. If you have only a few elements, you can just use an ArrayList. We recommend that you simply stay away from all methods that use an integer index to denote a position in a linked list. If you want random access into a collection, use an array or ArrayList, not a linked list. The program in Listing 9.1 puts linked lists to work. It simply creates two lists, merges them, then removes every second element from the second list, and finally tests the removeAll method. We recommend that you trace the program flow and pay special attention to the iterators. You may find it helpful to draw diagrams of the iterator positions, like this: |ACE |BDFG A|CE |BDFG AB|CE B|DFG . . . Note that the call System.out.println(a); prints all elements in the linked list a by invoking the toString method in AbstractCollection. From the Library of Hristo Dimov Hristov 504 Chapter 9 Collections Listing 9.1 linkedList/LinkedListTest.java 1 package linkedList; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 /** * This program demonstrates operations on linked lists. * @version 1.12 2018-04-10 * @author Cay Horstmann */ public class LinkedListTest { public static void main(String[] args) { var a = new LinkedList<String>(); a.add("Amy"); a.add("Carl"); a.add("Erica"); 18 19 20 21 22 23 var b = new LinkedList<String>(); b.add("Bob"); b.add("Doug"); b.add("Frances"); b.add("Gloria"); 24 25 // merge the words from b into a 26 27 28 ListIterator<String> aIter = a.listIterator(); Iterator<String> bIter = b.iterator(); 29 30 31 32 33 34 while (bIter.hasNext()) { if (aIter.hasNext()) aIter.next(); aIter.add(bIter.next()); } 35 36 System.out.println(a); 37 38 // remove every second word from b 39 40 41 42 43 44 45 bIter = b.iterator(); while (bIter.hasNext()) { bIter.next(); // skip one element if (bIter.hasNext()) { From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 505 bIter.next(); // skip next element bIter.remove(); // remove that element 46 47 } 48 } 49 50 System.out.println(b); 51 52 // bulk operation: remove all words in b from a 53 54 a.removeAll(b); 55 56 System.out.println(a); 57 } 58 59 } java.util.List<E> 1.2 • ListIterator<E> listIterator() returns a list iterator for visiting the elements of the list. • ListIterator<E> listIterator(int index) returns a list iterator for visiting the elements of the list whose first call to next will return the element with the given index. • void add(int i, E element) adds an element at the specified position. • void addAll(int i, Collection<? extends E> elements) adds all elements from a collection to the specified position. • E remove(int i) removes and returns the element at the specified position. • E get(int i) gets the element at the specified position. • E set(int i, E element) replaces the element at the specified position with a new element and returns the old element. • int indexOf(Object element) returns the position of the first occurrence of an element equal to the specified element, or -1 if no matching element is found. • int lastIndexOf(Object element) returns the position of the last occurrence of an element equal to the specified element, or -1 if no matching element is found. From the Library of Hristo Dimov Hristov 506 Chapter 9 Collections java.util.ListIterator<E> 1.2 • void add(E newElement) adds an element before the current position. • void set(E newElement) replaces the last element visited by next or previous with a new element. Throws an IllegalStateException if the list structure was modified since the last call to next or previous. • boolean hasPrevious() returns true if there is another element to visit when iterating backwards through the list. • E previous() returns the previous object. Throws a NoSuchElementException if the beginning of the list has been reached. • int nextIndex() returns the index of the element that would be returned by the next call to next. • int previousIndex() returns the index of the element that would be returned by the next call to previous. java.util.LinkedList<E> 1.2 • LinkedList() constructs an empty linked list. • LinkedList(Collection<? extends E> elements) constructs a linked list and adds all elements from a collection. • void addFirst(E element) • void addLast(E element) adds an element to the beginning or the end of the list. • E getFirst() • E getLast() returns the element at the beginning or the end of the list. • E removeFirst() • E removeLast() removes and returns the element at the beginning or the end of the list. From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 507 9.3.2 Array Lists In the preceding section, you saw the List interface and the LinkedList class that implements it. The List interface describes an ordered collection in which the position of elements matters. There are two protocols for visiting the elements: through an iterator and by random access with methods get and set. The latter is not appropriate for linked lists, but of course get and set make a lot of sense for arrays. The collections library supplies the familiar ArrayList class that also implements the List interface. An ArrayList encapsulates a dynamically reallocated array of objects. NOTE: If you are a veteran Java programmer, you may have used the Vector class whenever you need a dynamic array. Why use an ArrayList instead of a Vector? For one simple reason: All methods of the Vector class are synchronized. It is safe to access a Vector object from two threads. But if you access a vector from only a single thread—by far the more common case—your code wastes quite a bit of time with synchronization. In contrast, the ArrayList methods are not synchronized. We recommend that you use an ArrayList instead of a Vector whenever you don’t need synchronization. 9.3.3 Hash Sets Linked lists and arrays let you specify the order in which you want to arrange the elements. However, if you are looking for a particular element and don’t remember its position, you need to visit all elements until you find a match. That can be time consuming if the collection contains many elements. If you don’t care about the ordering of the elements, there are data structures that let you find elements much faster. The drawback is that those data structures give you no control over the order in which the elements appear. These data structures organize the elements in an order that is convenient for their own purposes. A well-known data structure for finding objects quickly is the hash table. A hash table computes an integer, called the hash code, for each object. A hash code is somehow derived from the instance fields of an object, preferably in such a way that objects with different data yield different codes. Table 9.2 lists a few examples of hash codes that result from the hashCode method of the String class. If you define your own classes, you are responsible for implementing your own hashCode method—see Chapter 5 for more information. Your implementation needs to be compatible with the equals method: If a.equals(b), then a and b must have the same hash code. From the Library of Hristo Dimov Hristov 508 Chapter 9 Collections Table 9.2 Hash Codes Resulting from the hashCode Method String Hash Code "Lee" 76268 "lee" 107020 "eel" 100300 What’s important for now is that hash codes can be computed quickly and that the computation depends only on the state of the object that needs to be hashed, not on the other objects in the hash table. In Java, hash tables are implemented as arrays of linked lists. Each list is called a bucket (see Figure 9.10). To find the place of an object in the table, compute its hash code and reduce it modulo the total number of buckets. The resulting number is the index of the bucket that holds the element. For example, if an object has hash code 76268 and there are 128 buckets, then the object is placed in bucket 108 (because the remainder 76268 % 128 is 108). Perhaps you are lucky and there is no other element in that bucket. Then, you simply insert the element into that bucket. Of course, sometimes you will hit a bucket that is already filled. This is called a hash collision. Then, compare the new object with all objects in that bucket to see if it is already present. If the hash codes are reasonably randomly distributed and the number of buckets is large enough, only a few comparisons should be necessary. Figure 9.10 A hash table From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 509 NOTE: As of Java 8, the buckets change from linked lists into balanced binary trees when they get full. This improves performance if a hash function was poorly chosen and yields many collisions, or if malicious code tries to flood a hash table with many values that have identical hash codes. If you want more control over the performance of the hash table, you can specify the initial bucket count. The bucket count gives the number of buckets used to collect objects with identical hash values. If too many elements are inserted into a hash table, the number of collisions increases and retrieval performance suffers. If you know how many elements, approximately, will eventually be in the table, you can set the bucket count. Typically, you should set it to somewhere between 75% and 150% of the expected element count. Some researchers believe that it is a good idea to make the bucket count a prime number to prevent a clustering of keys. The evidence for this isn’t conclusive, however. The standard library uses bucket counts that are powers of 2, with a default of 16. (Any value you supply for the table size is automatically rounded to the next power of 2.) Of course, you do not always know how many elements you need to store, or your initial guess may be too low. If the hash table gets too full, it needs to be rehashed. To rehash the table, a table with more buckets is created, all elements are inserted into the new table, and the original table is discarded. The load factor determines when a hash table is rehashed. For example, if the load factor is 0.75 (which is the default) and the table is more than 75% full, it is automatically rehashed with twice as many buckets. For most applications, it is reasonable to leave the load factor at 0.75. Hash tables can be used to implement several important data structures. The simplest among them is the set type. A set is a collection of elements without duplicates. The add method of a set first tries to find the object to be added, and adds it only if it is not yet present. The Java collections library supplies a HashSet class that implements a set based on a hash table. You add elements with the add method. The contains method is redefined to make a fast lookup to see if an element is already present in the set. It checks only the elements in one bucket and not all elements in the collection. The hash set iterator visits all buckets in turn. Since hashing scatters the elements around in the table, they are visited in a seemingly random order. You would only use a HashSet if you don’t care about the ordering of the elements in the collection. From the Library of Hristo Dimov Hristov 510 Chapter 9 Collections The sample program at the end of this section (Listing 9.2) reads words from System.in, adds them to a set, and finally prints out the first twenty words in the set. For example, you can feed the program the text from Alice in Wonderland (which you can obtain from www.gutenberg.org) by launching it from a command shell as java SetTest < alice30.txt The program reads all words from the input and adds them to the hash set. It then iterates through the unique words in the set and finally prints out a count. (Alice in Wonderland has 5,909 unique words, including the copyright notice at the beginning.) The words appear in random order. CAUTION: Be careful when you mutate set elements. If the hash code of an element were to change, the element would no longer be in the correct position in the data structure. Listing 9.2 set/SetTest.java 1 package set; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 /** * This program uses a set to print all unique words in System.in. * @version 1.12 2015-06-21 * @author Cay Horstmann */ public class SetTest { public static void main(String[] args) { var words = new HashSet<String>(); long totalTime = 0; 16 17 18 19 20 21 22 23 24 25 26 27 try (var in = new Scanner(System.in)) { while (in.hasNext()) { String word = in.next(); long callTime = System.currentTimeMillis(); words.add(word); callTime = System.currentTimeMillis() - callTime; totalTime += callTime; } } 28 From the Library of Hristo Dimov Hristov 9.3 511 Iterator<String> iter = words.iterator(); for (int i = 1; i <= 20 && iter.hasNext(); i++) System.out.println(iter.next()); System.out.println(". . ."); System.out.println(words.size() + " distinct words. " + totalTime + " milliseconds."); 29 30 31 32 33 } 34 35 Concrete Collections } java.util.HashSet<E> 1.2 • HashSet() constructs an empty hash set. • HashSet(Collection<? extends E> elements) constructs a hash set and adds all elements from a collection. • HashSet(int initialCapacity) constructs an empty hash set with the specified capacity (number of buckets). • HashSet(int initialCapacity, float loadFactor) constructs an empty hash set with the specified capacity and load factor. If the ratio size/capacity exceeds the load factor, the hash table will be rehashed into a larger one. java.lang.Object 1.0 • int hashCode() returns a hash code for this object. A hash code can be any integer, positive or negative. The definitions of equals and hashCode must be compatible: If x.equals(y) is true, then x.hashCode() must be the same value as y.hashCode(). 9.3.4 Tree Sets The TreeSet class is similar to the hash set, with one added improvement. A tree set is a sorted collection. You insert elements into the collection in any order. When you iterate through the collection, the values are automatically presented in sorted order. For example, suppose you insert three strings and then visit all elements that you added. var sorter = new TreeSet<String>(); sorter.add("Bob"); sorter.add("Amy"); sorter.add("Carl"); for (String s : sorter) System.out.println(s); From the Library of Hristo Dimov Hristov 512 Chapter 9 Collections Then, the values are printed in sorted order: Amy Bob Carl. As the name of the class suggests, the sorting is accomplished by a tree data structure. (The current implementation uses a red-black tree. For a detailed description of redblack trees see, for example, Introduction to Algorithms by Thomas Cormen, Charles Leiserson, Ronald Rivest, and Clifford Stein, The MIT Press, 2009.) Every time an element is added to a tree, it is placed into its proper sorting position. Therefore, the iterator always visits the elements in sorted order. Adding an element to a tree is slower than adding it to a hash table—see Table 9.3 for a comparison. But it is still much faster than checking for duplicates in an array or linked list. If the tree contains n elements, then an average of log2 n comparisons are required to find the correct position for the new element. For example, if the tree already contains 1,000 elements, adding a new element requires about 10 comparisons. NOTE: In order to use a tree set, you must be able to compare the elements. The elements must implement the Comparable interface, or you must supply a Comparator when constructing the set. (The Comparable and Comparator interfaces were introduced in Chapter 6.) Table 9.3 Adding Elements into Hash and Tree Sets Document Total Number of Words Number of Distinct Words HashSet TreeSet Alice in Wonderland 28195 5909 5 sec 7 sec The Count of Monte Cristo 466300 37545 75 sec 98 sec If you look back at Table 9.3, you may well wonder if you should always use a tree set instead of a hash set. After all, adding elements does not seem to take much longer, and the elements are automatically sorted. The answer depends on the data that you are collecting. If you don’t need the data sorted, there is no reason to pay for the sorting overhead. More important, with some data it is much more difficult to come up with a sort order than a hash function. A hash function only needs to do a reasonably good job of scrambling the objects, whereas a comparison function must tell objects apart with complete precision. To make this distinction more concrete, consider the task of collecting a set of rectangles. If you use a TreeSet, you need to supply a Comparator<Rectangle>. How do you compare two rectangles? By area? That doesn’t work. You can have From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 513 two different rectangles with different coordinates but the same area. The sort order for a tree must be a total ordering. Any two elements must be comparable, and the comparison can only be zero if the elements are equal. There is such a sort order for rectangles (the lexicographic ordering on its coordinates), but it is unnatural and cumbersome to compute. In contrast, a hash function is already defined for the Rectangle class. It simply hashes the coordinates. NOTE: As of Java 6, the TreeSet class implements the NavigableSet interface. That interface adds several convenient methods for locating elements and for backward traversal. See the API notes for details. The program in Listing 9.3 builds two tree sets of Item objects. The first one is sorted by part number, the default sort order of Item objects. The second set is sorted by description, using a custom comparator. Listing 9.3 treeSet/TreeSetTest.java 1 package treeSet; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 /** * This program sorts a set of Item objects by comparing their descriptions. * @version 1.13 2018-04-10 * @author Cay Horstmann */ public class TreeSetTest { public static void main(String[] args) { var parts = new TreeSet<Item>(); parts.add(new Item("Toaster", 1234)); parts.add(new Item("Widget", 4562)); parts.add(new Item("Modem", 9912)); System.out.println(parts); 19 var sortByDescription = new TreeSet<Item>(Comparator.comparing(Item::getDescription)); 20 21 sortByDescription.addAll(parts); System.out.println(sortByDescription); 22 23 } 24 25 } From the Library of Hristo Dimov Hristov 514 Chapter 9 Collections Listing 9.4 treeSet/Item.java 1 package treeSet; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 /** * An item with a description and a part number. */ public class Item implements Comparable<Item> { private String description; private int partNumber; 12 13 14 15 16 17 18 19 20 21 22 /** * Constructs an item. * @param aDescription the item's description * @param aPartNumber the item's part number */ public Item(String aDescription, int aPartNumber) { description = aDescription; partNumber = aPartNumber; } 23 24 25 26 27 28 29 30 31 /** * Gets the description of this item. * @return the description */ public String getDescription() { return description; } 32 33 34 35 36 public String toString() { return "[description=" + description + ", partNumber=" + partNumber + "]"; } 37 38 39 40 41 public boolean equals(Object otherObject) { if (this == otherObject) return true; if (otherObject == null) return false; From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 515 if (getClass() != otherObject.getClass()) return false; var other = (Item) otherObject; return Objects.equals(description, other.description) && partNumber == other.partNumber; 42 43 44 } 45 46 public int hashCode() { return Objects.hash(description, partNumber); } 47 48 49 50 51 public int compareTo(Item other) { int diff = Integer.compare(partNumber, other.partNumber); return diff != 0 ? diff : description.compareTo(other.description); } 52 53 54 55 56 57 } java.util.TreeSet<E> 1.2 • TreeSet() • TreeSet(Comparator<? super E> comparator) constructs an empty tree set. • TreeSet(Collection<? extends E> elements) • TreeSet(SortedSet<E> s) constructs a tree set and adds all elements from a collection or sorted set (in the latter case, using the same ordering). java.util.SortedSet<E> 1.2 • Comparator<? super E> comparator() returns the comparator used for sorting the elements, or null if the elements are compared with the compareTo method of the Comparable interface. • E first() • E last() returns the smallest or largest element in the sorted set. From the Library of Hristo Dimov Hristov 516 Chapter 9 Collections java.util.NavigableSet<E> 6 • E higher(E value) • E lower(E value) returns the least element > value or the largest element < value, or null if there is no such element. • E ceiling(E value) • E floor(E value) returns the least element ≥ value or the largest element ≤ value, or null if there is no such element. • E pollFirst() • E pollLast() removes and returns the smallest or largest element in this set, or null if the set is empty. • Iterator<E> descendingIterator() returns an iterator that traverses this set in descending direction. 9.3.5 Queues and Deques As we already discussed, a queue lets you efficiently add elements at the tail and remove elements from the head. A double-ended queue, or deque, lets you efficiently add or remove elements at the head and tail. Adding elements in the middle is not supported. Java 6 introduced a Deque interface. It is implemented by the ArrayDeque and LinkedList classes, both of which provide deques whose size grows as needed. In Chapter 12, you will see bounded queues and deques. java.util.Queue<E> 5 • boolean add(E element) • boolean offer(E element) adds the given element to the tail of this queue and returns true, provided the queue is not full. If the queue is full, the first method throws an IllegalStateException, whereas the second method returns false. (Continues) From the Library of Hristo Dimov Hristov 9.3 Concrete Collections 517 java.util.Queue<E> 5 (Continued) • E remove() • E poll() removes and returns the element at the head of this queue, provided the queue is not empty. If the queue is empty, the first method throws a NoSuchElementException, whereas the second method returns null. • E element() • E peek() returns the element at the head of this queue without removing it, provided the queue is not empty. If the queue is empty, the first method throws a NoSuchElementException, whereas the second method returns null. java.util.Deque<E> 6 • • • • void addFirst(E element) void addLast(E element) boolean offerFirst(E element) boolean offerLast(E element) adds the given element to the head or tail of this deque. If the deque is full, the first two methods throw an IllegalStateException, whereas the last two methods return false. • • • • E E E E removeFirst() removeLast() pollFirst() pollLast() removes and returns the element at the head of this deque, provided the deque is not empty. If the deque is empty, the first two methods throw a NoSuchElementException, whereas the last two methods return null. • • • • E E E E getFirst() getLast() peekFirst() peekLast() returns the element at the head of this deque without removing it, provided the deque is not empty. If the deque is empty, the first two methods throw a NoSuchElementException, whereas the last two methods return null. From the Library of Hristo Dimov Hristov 518 Chapter 9 Collections java.util.ArrayDeque<E> 6 • ArrayDeque() • ArrayDeque(int initialCapacity) constructs an unbounded deque with an initial capacity of 16 or the given initial capacity. 9.3.6 Priority Queues A priority queue retrieves elements in sorted order after they were inserted in arbitrary order. That is, whenever you call the remove method, you get the smallest element currently in the priority queue. However, the priority queue does not sort all its elements. If you iterate over the elements, they are not necessarily sorted. The priority queue makes use of an elegant and efficient data structure called a heap. A heap is a self-organizing binary tree in which the add and remove operations cause the smallest element to gravitate to the root, without wasting time on sorting all elements. Just like a TreeSet, a priority queue can either hold elements of a class that implements the Comparable interface or a Comparator object you supply in the constructor. A typical use for a priority queue is job scheduling. Each job has a priority. Jobs are added in random order. Whenever a new job can be started, the highest priority job is removed from the queue. (Since it is traditional for priority 1 to be the “highest” priority, the remove operation yields the minimum element.) Listing 9.5 shows a priority queue in action. Unlike iteration in a TreeSet, the iteration here does not visit the elements in sorted order. However, removal always yields the smallest remaining element. Listing 9.5 priorityQueue/PriorityQueueTest.java 1 package priorityQueue; 2 3 4 import java.util.*; import java.time.*; 5 6 7 8 9 10 /** * This program demonstrates the use of a priority queue. * @version 1.02 2015-06-20 * @author Cay Horstmann */ From the Library of Hristo Dimov Hristov 9.4 11 12 13 14 15 16 17 18 19 Maps 519 public class PriorityQueueTest { public static void main(String[] args) { var pq = new PriorityQueue<LocalDate>(); pq.add(LocalDate.of(1906, 12, 9)); // G. Hopper pq.add(LocalDate.of(1815, 12, 10)); // A. Lovelace pq.add(LocalDate.of(1903, 12, 3)); // J. von Neumann pq.add(LocalDate.of(1910, 6, 22)); // K. Zuse 20 System.out.println("Iterating over elements . . ."); for (LocalDate date : pq) System.out.println(date); System.out.println("Removing elements . . ."); while (!pq.isEmpty()) System.out.println(pq.remove()); 21 22 23 24 25 26 } 27 28 } java.util.PriorityQueue 5 • PriorityQueue() • PriorityQueue(int initialCapacity) constructs a priority queue for storing Comparable objects. • PriorityQueue(int initialCapacity, Comparator<? super E> c) constructs a priority queue and uses the specified comparator for sorting its elements. 9.4 Maps A set is a collection that lets you quickly find an existing element. However, to look up an element, you need to have an exact copy of the element to find. That isn’t a very common lookup—usually, you have some key information, and you want to look up the associated element. The map data structure serves that purpose. A map stores key/value pairs. You can find a value if you provide the key. For example, you may store a table of employee records, where the keys are the employee IDs and the values are Employee objects. In the following sections, you will learn how to work with maps. 9.4.1 Basic Map Operations The Java library supplies two general-purpose implementations for maps: HashMap and TreeMap. Both classes implement the Map interface. From the Library of Hristo Dimov Hristov 520 Chapter 9 Collections A hash map hashes the keys, and a tree map uses an ordering on the keys to organize them in a search tree. The hash or comparison function is applied only to the keys. The values associated with the keys are not hashed or compared. Should you choose a hash map or a tree map? As with sets, hashing is usually a bit faster, and it is the preferred choice if you don’t need to visit the keys in sorted order. Here is how you set up a hash map for storing employees: var staff = new HashMap<String, Employee>(); // HashMap implements Map var harry = new Employee("Harry Hacker"); staff.put("987-98-9996", harry); . . . Whenever you add an object to a map, you must supply a key as well. In our case, the key is a string, and the corresponding value is an Employee object. To retrieve an object, you must use (and, therefore, remember) the key. var id = "987-98-9996"; Employee e = staff.get(id); // gets harry If no information is stored in the map with the particular key specified, get returns null. The null return value can be inconvenient. Sometimes, you have a good default that can be used for keys that are not present in the map. Then use the getOrDefault method. Map<String, Integer> scores = . . .; int score = scores.getOrDefault(id, 0); // gets 0 if the id is not present Keys must be unique. You cannot store two values with the same key. If you call the put method twice with the same key, the second value replaces the first one. In fact, put returns the previous value associated with its key parameter. The remove method removes an element with a given key from the map. The size method returns the number of entries in the map. The easiest way of iterating over the keys and values of a map is the forEach method. Provide a lambda expression that receives a key and a value. That expression is invoked for each map entry in turn. scores.forEach((k, v) -> System.out.println("key=" + k + ", value=" + v)); From the Library of Hristo Dimov Hristov 9.4 Maps 521 Listing 9.6 illustrates a map at work. We first add key/value pairs to a map. Then, we remove one key from the map, which removes its associated value as well. Next, we change the value that is associated with a key and call the get method to look up a value. Finally, we iterate through the entry set. Listing 9.6 map/MapTest.java 1 package map; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 /** * This program demonstrates the use of a map with key type String and value type Employee. * @version 1.12 2015-06-21 * @author Cay Horstmann */ public class MapTest { public static void main(String[] args) { var staff = new HashMap<String, Employee>(); staff.put("144-25-5464", new Employee("Amy Lee")); staff.put("567-24-2546", new Employee("Harry Hacker")); staff.put("157-62-7935", new Employee("Gary Cooper")); staff.put("456-62-5527", new Employee("Francesca Cruz")); 19 20 // print all entries 21 22 System.out.println(staff); 23 24 // remove an entry 25 26 staff.remove("567-24-2546"); 27 28 // replace an entry 29 30 staff.put("456-62-5527", new Employee("Francesca Miller")); 31 32 // look up a value 33 34 System.out.println(staff.get("157-62-7935")); 35 36 // iterate through all entries 37 (Continues) From the Library of Hristo Dimov Hristov 522 Chapter 9 Collections Listing 9.6 (Continued) staff.forEach((k, v) -> System.out.println("key=" + k + ", value=" + v)); 38 39 } 40 41 } java.util.Map<K, V> 1.2 • V get(Object key) gets the value associated with the key; returns the object associated with the key, or null if the key is not found in the map. Implementing classes may forbid null keys. • default V getOrDefault(Object key, V defaultValue) gets the value associated with the key; returns the object associated with the key, or defaultValue if the key is not found in the map. • V put(K key, V value) puts the association of a key and a value into the map. If the key is already present, the new object replaces the old one previously associated with the key. This method returns the old value of the key, or null if the key was not previously present. Implementing classes may forbid null keys or values. • void putAll(Map<? extends K, ? extends V> entries) adds all entries from the specified map to this map. • boolean containsKey(Object key) returns true if the key is present in the map. • boolean containsValue(Object value) returns true if the value is present in the map. • default void forEach(BiConsumer<? super K,? super V> action) 8 applies the action to all key/value pairs of this map. java.util.HashMap<K, V> 1.2 • HashMap() • HashMap(int initialCapacity) • HashMap(int initialCapacity, float loadFactor) constructs an empty hash map with the specified capacity and load factor (a number between 0.0 and 1.0 that determines at what percentage of fullness the hash table will be rehashed into a larger one). The default load factor is 0.75. From the Library of Hristo Dimov Hristov 9.4 Maps 523 java.util.TreeMap<K,V> 1.2 • TreeMap() constructs an empty tree map for keys that implement the Comparable interface. • TreeMap(Comparator<? super K> c) constructs a tree map and uses the specified comparator for sorting its keys. • TreeMap(Map<? extends K, ? extends V> entries) constructs a tree map and adds all entries from a map. • TreeMap(SortedMap<? extends K, ? extends V> entries) constructs a tree map, adds all entries from a sorted map, and uses the same element comparator as the given sorted map. java.util.SortedMap<K, V> 1.2 • Comparator<? super K> comparator() returns the comparator used for sorting the keys, or null if the keys are compared with the compareTo method of the Comparable interface. • K firstKey() • K lastKey() returns the smallest or largest key in the map. 9.4.2 Updating Map Entries A tricky part of dealing with maps is updating an entry. Normally, you get the old value associated with a key, update it, and put back the updated value. But you have to worry about the special case of the first occurrence of a key. Consider using a map for counting how often a word occurs in a file. When we see a word, we’d like to increment a counter like this: counts.put(word, counts.get(word) + 1); That works, except in the case when word is encountered for the first time. Then get returns null, and a NullPointerException occurs. A simple remedy is to use the getOrDefault method: counts.put(word, counts.getOrDefault(word, 0) + 1); Another approach is to first call the putIfAbsent method. It only puts a value if the key was previously absent (or mapped to null). From the Library of Hristo Dimov Hristov 524 Chapter 9 Collections counts.putIfAbsent(word, 0); counts.put(word, counts.get(word) + 1); // now we know that get will succeed But you can do better than that. The merge method simplifies this common operation. The call counts.merge(word, 1, Integer::sum); associates word with 1 if the key wasn’t previously present, and otherwise combines the previous value and 1, using the Integer::sum function. The API notes describe other methods for updating map entries that are less commonly used. java.util.Map<K, V> 1.2 • default V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction) 8 If key is associated with a non-null value v, applies the function to v and value and either associates key with the result or, if the result is null, removes the key. Otherwise, associates key with value. Returns get(key). • default V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) 8 Applies the function to key and get(key). Either associates key with the result or, if the result is null, removes the key. Returns get(key). • default V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) 8 If key is associated with a non-null value v, applies the function to key and v and either associates key with the result or, if the result is null, removes the key. Returns get(key). • default V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) 8 Applies the function to key unless key is associated with a non-null value. Either associates key with the result or, if the result is null, removes the key. Returns get(key). • default void replaceAll(BiFunction<? super K,? super V,? extends V> function) 8 Calls the function on all entries. Associates keys with non-null results and removes keys with null results. • default V putIfAbsent(K key, V value) 8 If key is absent or associated with null, associates it with value and returns null. Otherwise returns the associated value. From the Library of Hristo Dimov Hristov 9.4 Maps 525 9.4.3 Map Views The collections framework does not consider a map itself as a collection. (Other frameworks for data structures consider a map as a collection of key/value pairs, or as a collection of values indexed by the keys.) However, you can obtain views of the map—objects that implement the Collection interface or one of its subinterfaces. There are three views: the set of keys, the collection of values (which is not a set), and the set of key/value pairs. The keys and key/value pairs form a set because there can be only one copy of a key in a map. The methods Set<K> keySet() Collection<V> values() Set<Map.Entry<K, V>> entrySet() return these three views. (The elements of the entry set are objects of a class implementing the Map.Entry interface.) Note that the keySet is not a HashSet or TreeSet, but an object of some other class that implements the Set interface. The Set interface extends the Collection interface. Therefore, you can use a keySet as you would use any collection. For example, you can enumerate all keys of a map: Set<String> keys = map.keySet(); for (String key : keys) { do something with key } If you want to look at both keys and values, you can avoid value lookups by enumerating the entries. Use the following code skeleton: for (Map.Entry<String, Employee> entry : staff.entrySet()) { String k = entry.getKey(); Employee v = entry.getValue(); do something with k, v } TIP: You can avoid the cumbersome Map.Entry by using a var declaration. for (var entry : map.entrySet()) { do something with entry.getKey(), entry.getValue() } Or simply use the forEach method: From the Library of Hristo Dimov Hristov 526 Chapter 9 Collections map.forEach((k, v) -> { do something with k, v }); If you invoke the remove method of the iterator on the key set view, you actually remove the key and its associated value from the map. However, you cannot add an element to the key set view. It makes no sense to add a key without also adding a value. If you try to invoke the add method, it throws an UnsupportedOperationException. The entry set view has the same restriction, even though it would make conceptual sense to add a new key/value pair. java.util.Map<K, V> 1.2 • Set<Map.Entry<K, V>> entrySet() returns a set view of Map.Entry objects, the key/value pairs in the map. You can remove elements from this set and they are removed from the map, but you cannot add any elements. • Set<K> keySet() returns a set view of all keys in the map. You can remove elements from this set and the keys and associated values are removed from the map, but you cannot add any elements. • Collection<V> values() returns a collection view of all values in the map. You can remove elements from this set and the removed value and its key are removed from the map, but you cannot add any elements. java.util.Map.Entry<K, V> 1.2 • K getKey() • V getValue() returns the key or value of this entry. • V setValue(V newValue) changes the value in the associated map to the new value and returns the old value. 9.4.4 Weak Hash Maps The collection class library has several map classes for specialized needs that we briefly discuss in this and the following sections. From the Library of Hristo Dimov Hristov 9.4 Maps 527 The WeakHashMap class was designed to solve an interesting problem. What happens with a value whose key is no longer used anywhere in your program? Suppose the last reference to a key has gone away. Then, there is no longer any way to refer to the value object. But, as no part of the program has the key any more, the key/value pair cannot be removed from the map. Why can’t the garbage collector remove it? Isn’t it the job of the garbage collector to remove unused objects? Unfortunately, it isn’t quite so simple. The garbage collector traces live objects. As long as the map object is live, all buckets in it are live and won’t be reclaimed. Thus, your program should take care to remove unused values from long-lived maps. Or, you can use a WeakHashMap instead. This data structure cooperates with the garbage collector to remove key/value pairs when the only reference to the key is the one from the hash table entry. Here are the inner workings of this mechanism. The WeakHashMap uses weak references to hold keys. A WeakReference object holds a reference to another object—in our case, a hash table key. Objects of this type are treated in a special way by the garbage collector. Normally, if the garbage collector finds that a particular object has no references to it, it simply reclaims the object. However, if the object is reachable only by a WeakReference, the garbage collector still reclaims the object, but places the weak reference that led to it into a queue. The operations of the WeakHashMap periodically check that queue for newly arrived weak references. The arrival of a weak reference in the queue signifies that the key was no longer used by anyone and has been collected. The WeakHashMap then removes the associated entry. 9.4.5 Linked Hash Sets and Maps The LinkedHashSet and LinkedHashMap classes remember in which order you inserted items. That way, you can avoid the seemingly random order of items in a hash table. As entries are inserted into the table, they are joined in a doubly linked list (see Figure 9.11). For example, consider the following map insertions from Listing 9.6: var staff = new LinkedHashMap<String, Employee>(); staff.put("144-25-5464", new Employee("Amy Lee")); staff.put("567-24-2546", new Employee("Harry Hacker")); staff.put("157-62-7935", new Employee("Gary Cooper")); staff.put("456-62-5527", new Employee("Francesca Cruz")); Then, staff.keySet().iterator() enumerates the keys in this order: 144-25-5464 567-24-2546 From the Library of Hristo Dimov Hristov 528 Chapter 9 Collections Figure 9.11 A linked hash table 157-62-7935 456-62-5527 and staff.values().iterator() enumerates the values in this order: Amy Lee Harry Hacker Gary Cooper Francesca Cruz A linked hash map can alternatively use access order, not insertion order, to iterate through the map entries. Every time you call get or put, the affected entry is removed from its current position and placed at the end of the linked list of entries. (Only the position in the linked list of entries is affected, not the hash table bucket. An entry always stays in the bucket that corresponds to the hash code of the key.) To construct such a hash map, call LinkedHashMap<K, V>(initialCapacity, loadFactor, true) Access order is useful for implementing a “least recently used” discipline for a cache. For example, you may want to keep frequently accessed entries in memory and read less frequently accessed objects from a database. When you don’t find an entry in the table, and the table is already pretty full, you can get an iterator into the table and remove the first few elements that it enumerates. Those entries were the least recently used ones. You can even automate that process. Form a subclass of LinkedHashMap and override the method From the Library of Hristo Dimov Hristov 9.4 Maps 529 protected boolean removeEldestEntry(Map.Entry<K, V> eldest) Adding a new entry then causes the eldest entry to be removed whenever your method returns true. For example, the following cache is kept at a size of at most 100 elements: var cache = new LinkedHashMap<K, V>(128, 0.75F, true) { protected boolean removeEldestEntry(Map.Entry<K, V> eldest) { return size() > 100; } }; Alternatively, you can consider the eldest entry to decide whether to remove it. For example, you may want to check a time stamp stored with the entry. 9.4.6 Enumeration Sets and Maps The EnumSet is an efficient set implementation with elements that belong to an enumerated type. Since an enumerated type has a finite number of instances, the EnumSet is internally implemented simply as a sequence of bits. A bit is turned on if the corresponding value is present in the set. The EnumSet class has no public constructors. Use a static factory method to construct the set: enum Weekday { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }; EnumSet<Weekday> always = EnumSet.allOf(Weekday.class); EnumSet<Weekday> never = EnumSet.noneOf(Weekday.class); EnumSet<Weekday> workday = EnumSet.range(Weekday.MONDAY, Weekday.FRIDAY); EnumSet<Weekday> mwf = EnumSet.of(Weekday.MONDAY, Weekday.WEDNESDAY, Weekday.FRIDAY); You can use the usual methods of the Set interface to modify an EnumSet. An EnumMap is a map with keys that belong to an enumerated type. It is simply and efficiently implemented as an array of values. You need to specify the key type in the constructor: var personInCharge = new EnumMap<Weekday, Employee>(Weekday.class); NOTE: In the API documentation for EnumSet, you will see odd-looking type parameters of the form E extends Enum<E>. This simply means “E is an enumerated type.” All enumerated types extend the generic Enum class. For example, Weekday extends Enum<Weekday>. From the Library of Hristo Dimov Hristov 530 Chapter 9 Collections 9.4.7 Identity Hash Maps The IdentityHashMap has a quite specialized purpose. Here, the hash values for the keys should not be computed by the hashCode method but by the System.identityHashCode method. That’s the method that Object.hashCode uses to compute a hash code from the object’s memory address. Also, for comparison of objects, the IdentityHashMap uses ==, not equals. In other words, different key objects are considered distinct even if they have equal contents. This class is useful for implementing object traversal algorithms, such as object serialization, in which you want to keep track of which objects have already been traversed. java.util.WeakHashMap<K, V> 1.2 • WeakHashMap() • WeakHashMap(int initialCapacity) • WeakHashMap(int initialCapacity, float loadFactor) constructs an empty hash map with the specified capacity and load factor. java.util.LinkedHashSet<E> 1.4 • LinkedHashSet() • LinkedHashSet(int initialCapacity) • LinkedHashSet(int initialCapacity, float loadFactor) constructs an empty linked hash set with the specified capacity and load factor. java.util.LinkedHashMap<K, V> 1.4 • • • • LinkedHashMap() LinkedHashMap(int initialCapacity) LinkedHashMap(int initialCapacity, float loadFactor) LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) constructs an empty linked hash map with the specified capacity, load factor, and ordering. The accessOrder parameter is true for access order, false for insertion order. (Continues) From the Library of Hristo Dimov Hristov 9.4 Maps 531 java.util.LinkedHashMap<K, V> 1.4 (Continued) • protected boolean removeEldestEntry(Map.Entry<K, V> eldest) should be overridden to return true if you want the eldest entry to be removed. The eldest parameter is the entry whose removal is being contemplated. This method is called after an entry has been added to the map. The default implementation returns false—old elements are not removed by default. However, you can redefine this method to selectively return true—for example, if the eldest entry fits a certain condition or if the map exceeds a certain size. java.util.EnumSet<E extends Enum<E>> 5 • static <E extends Enum<E>> EnumSet<E> allOf(Class<E> enumType) returns a mutable set that contains all values of the given enumerated type. • static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> enumType) returns a mutable set that is initially empty. • static <E extends Enum<E>> EnumSet<E> range(E from, E to) returns a mutable set that contains all values between from and to (inclusive). • static <E extends Enum<E>> EnumSet<E> of(E e) . . . • static <E extends Enum<E>> EnumSet<E> of(E e1, E e2, E e3, E e4, E e5) • static <E extends Enum<E>> EnumSet<E> of(E first, E... rest) returns a mutable set containing the given elements which must not be null. java.util.EnumMap<K extends Enum<K>, V> 5 • EnumMap(Class<K> keyType) constructs an empty mutable map whose keys have the given type. java.util.IdentityHashMap<K, V> 1.4 • IdentityHashMap() • IdentityHashMap(int expectedMaxSize) constructs an empty identity hash map whose capacity is the smallest power of 2 exceeding 1.5 × expectedMaxSize. (The default for expectedMaxSize is 21.) From the Library of Hristo Dimov Hristov 532 Chapter 9 Collections java.lang.System 1.0 • static int identityHashCode(Object obj) 1.1 returns the same hash code (derived from the object’s memory address) that Object.hashCode computes, even if the class to which obj belongs has redefined the hashCode method. 9.5 Views and Wrappers If you look at Figures 9.4 and 9.5, you might think it is overkill to have lots of interfaces and abstract classes to implement a modest number of concrete collection classes. However, these figures don’t tell the whole story. By using views, you can obtain other objects that implement the Collection or Map interfaces. You saw one example of this with the keySet method of the map classes. At first glance, it appears as if the method creates a new set, fills it with all the keys of the map, and returns it. However, that is not the case. Instead, the keySet method returns an object of a class that implements the Set interface and whose methods manipulate the original map. Such a collection is called a view. The technique of views has a number of useful applications in the collections framework. We will discuss these applications in the following sections. 9.5.1 Small Collections Java 9 introduces static methods yielding a set or list with given elements, and a map with given key/value pairs. For example, List<String> names = List.of("Peter", "Paul", "Mary"); Set<Integer> numbers = Set.of(2, 3, 5); yield a list and a set with three elements. For a map, you specify the keys and values, like this: Map<String, Integer> scores = Map.of("Peter", 2, "Paul", 3, "Mary", 5); The elements, keys, or values may not be null. The List and Set interfaces have eleven of methods with zero to ten arguments, and an of method with a variable number of arguments. The specializations are provided for efficiency. From the Library of Hristo Dimov Hristov 9.5 Views and Wrappers 533 For the Map interface, it is not possible to provide a version with variable arguments since the argument types alternate between the key and value types. There is a static method ofEntries that accepts an arbitrary number of Map.Entry<K, V> objects, which you can create with the static entry method. For example, import static java.util.Map.*; . . . Map<String, Integer> scores = ofEntries( entry("Peter", 2), entry("Paul", 3), entry("Mary", 5)); The of and ofEntries methods produce objects of classes that have an instance variable for each element, or that are backed by an array. These collection objects are unmodifiable. Any attempt to change their contents results in an UnsupportedOperationException. If you want a mutable collection, you can pass the unmodifiable collection to the constructor: var names = new ArrayList<>(List.of("Peter", "Paul", "Mary")); The method call Collections.nCopies(n, anObject) returns an immutable object that implements the List interface and gives the illusion of having n elements, each of which appears as anObject. For example, the following call creates a List containing 100 strings, all set to "DEFAULT": List<String> settings = Collections.nCopies(100, "DEFAULT"); There is very little storage cost—the object is stored only once. NOTE: The of methods were introduced in Java 9. Previously, there was a static Arrays.asList method that returns a list that is mutable but not resizable. That is, you can call set but not add or remove on the list. There are also legacy methods Collections.emptySet and Collections.singleton. NOTE: The Collections class contains a number of utility methods with parameters or return values that are collections. Do not confuse it with the Collection interface. From the Library of Hristo Dimov Hristov 534 Chapter 9 Collections TIP: Java doesn’t have a Pair class, and some programmers use a Map.Entry as a poor man’s pair. Before Java 9, this was painful—you had to construct a new AbstractMap.SimpleImmutableEntry<>(first, second). Nowadays, you can call Map.entry(first, second). 9.5.2 Subranges You can form subrange views for a number of collections. For example, suppose you have a list staff and want to extract elements 10 to 19. Use the subList method to obtain a view into the subrange of the list: List<Employee> group2 = staff.subList(10, 20); The first index is inclusive, the second exclusive—just like the parameters for the substring operation of the String class. You can apply any operations to the subrange, and they automatically reflect the entire list. For example, you can erase the entire subrange: group2.clear(); // staff reduction The elements get automatically cleared from the staff list, and group2 becomes empty. For sorted sets and maps, you use the sort order, not the element position, to form subranges. The SortedSet interface declares three methods: SortedSet<E> subSet(E from, E to) SortedSet<E> headSet(E to) SortedSet<E> tailSet(E from) These return the subsets of all elements that are larger than or equal to from and strictly smaller than to. For sorted maps, the similar methods SortedMap<K, V> subMap(K from, K to) SortedMap<K, V> headMap(K to) SortedMap<K, V> tailMap(K from) return views into the maps consisting of all entries in which the keys fall into the specified ranges. The NavigableSet interface introduced in Java 6 gives more control over these subrange operations. You can specify whether the bounds are included: NavigableSet<E> subSet(E from, boolean fromInclusive, E to, boolean toInclusive) NavigableSet<E> headSet(E to, boolean toInclusive) NavigableSet<E> tailSet(E from, boolean fromInclusive) From the Library of Hristo Dimov Hristov 9.5 Views and Wrappers 535 9.5.3 Unmodifiable Views The Collections class has methods that produce unmodifiable views of collections. These views add a runtime check to an existing collection. If an attempt to modify the collection is detected, an exception is thrown and the collection remains untouched. You obtain unmodifiable views by eight methods: Collections.unmodifiableCollection Collections.unmodifiableList Collections.unmodifiableSet Collections.unmodifiableSortedSet Collections.unmodifiableNavigableSet Collections.unmodifiableMap Collections.unmodifiableSortedMap Collections.unmodifiableNavigableMap Each method is defined to work on an interface. For example, Collections .unmodifiableList works with an ArrayList, a LinkedList, or any other class that implements the List interface. For example, suppose you want to let some part of your code look at, but not touch, the contents of a collection. Here is what you could do: var staff = new LinkedList<String>(); . . . lookAt(Collections.unmodifiableList(staff)); The Collections.unmodifiableList method returns an object of a class implementing the List interface. Its accessor methods retrieve values from the staff collection. Of course, the lookAt method can call all methods of the List interface, not just the accessors. But all mutator methods (such as add) have been redefined to throw an UnsupportedOperationException instead of forwarding the call to the underlying collection. The unmodifiable view does not make the collection itself immutable. You can still modify the collection through its original reference (staff, in our case). And you can still call mutator methods on the elements of the collection. The views wrap the interface and not the actual collection object, so you only have access to those methods that are defined in the interface. For example, the LinkedList class has convenience methods, addFirst and addLast, that are not part of the List interface. These methods are not accessible through the unmodifiable view. From the Library of Hristo Dimov Hristov 536 Chapter 9 Collections CAUTION: The unmodifiableCollection method (as well as the synchronizedCollection and checkedCollection methods discussed later in this section) returns a collection whose equals method does not invoke the equals method of the underlying collection. Instead, it inherits the equals method of the Object class, which just tests whether the objects are identical. If you turn a set or list into just a collection, you can no longer test for equal contents. The view acts in this way because equality testing is not well defined at this level of the hierarchy. The views treat the hashCode method in the same way. However, the unmodifiableSet and unmodifiableList methods use the equals and hashCode methods of the underlying collections. 9.5.4 Synchronized Views If you access a collection from multiple threads, you need to ensure that the collection is not accidentally damaged. For example, it would be disastrous if one thread tried to add to a hash table while another thread was rehashing the elements. Instead of implementing thread-safe collection classes, the library designers used the view mechanism to make regular collections thread safe. For example, the static synchronizedMap method in the Collections class can turn any map into a Map with synchronized access methods: var map = Collections.synchronizedMap(new HashMap<String, Employee>()); You can now access the map object from multiple threads. The methods such as get and put are synchronized—each method call must be finished completely before another thread can call another method. We discuss the issue of synchronized access to data structures in greater detail in Chapter 12. 9.5.5 Checked Views Checked views are intended as debugging support for a problem that can occur with generic types. As explained in Chapter 8, it is actually possible to smuggle elements of the wrong type into a generic collection. For example: var strings = new ArrayList<String>(); ArrayList rawList = strings; // warning only, not an error, // for compatibility with legacy code rawList.add(new Date()); // now strings contains a Date object! The erroneous add command is not detected at runtime. Instead, a class cast exception will happen later when another part of the code calls get and casts the result to a String. From the Library of Hristo Dimov Hristov 9.5 Views and Wrappers 537 A checked view can detect this problem. Define a safe list as follows: List<String> safeStrings = Collections.checkedList(strings, String.class); The view’s add method checks that the inserted object belongs to the given class and immediately throws a ClassCastException if it does not. The advantage is that the error is reported at the correct location: ArrayList rawList = safeStrings; rawList.add(new Date()); // checked list throws a ClassCastException CAUTION: The checked views are limited by the runtime checks that the virtual machine can carry out. For example, if you have an ArrayList<Pair<String>>, you cannot protect it from inserting a Pair<Date> since the virtual machine has a single “raw” Pair class. 9.5.6 A Note on Optional Operations A view usually has some restriction—it may be read-only, it may not be able to change the size, or it may support removal but not insertion (as is the case for the key view of a map). A restricted view throws an UnsupportedOperationException if you attempt an inappropriate operation. In the API documentation for the collection and iterator interfaces, many methods are described as “optional operations.” This seems to be in conflict with the notion of an interface. After all, isn’t the purpose of an interface to lay out the methods that a class must implement? Indeed, this arrangement is unsatisfactory from a theoretical perspective. A better solution might have been to design separate interfaces for read-only views and views that can’t change the size of a collection. However, that would have tripled the number of interfaces, which the designers of the library found unacceptable. Should you extend the technique of “optional” methods to your own designs? We think not. Even though collections are used frequently, the coding style for implementing them is not typical for other problem domains. The designers of a collection class library have to resolve a particularly brutal set of conflicting requirements. Users want the library to be easy to learn, convenient to use, completely generic, idiot-proof, and at the same time as efficient as hand-coded algorithms. It is plainly impossible to achieve all these goals simultaneously, or even to come close. But in your own programming problems, you will rarely encounter such an extreme set of constraints. You should be able to find solutions that do not rely on the extreme measure of “optional” interface operations. From the Library of Hristo Dimov Hristov 538 Chapter 9 Collections java.util.List 1.2 • static <E> List<E> of() 9 • static <E> List<E> of(E e1) 9 . . . • static <E> List<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10) 9 • static <E> List<E> of(E... elements) 9 yields an immutable list of the given elements, which must not be null. java.util.Set 1.2 • static <E> Set<E> of() 9 • static <E> Set<E> of(E e1) 9 . . . • static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10) 9 • static <E> Set<E> of(E... elements) 9 yields an immutable set of the given elements, which must not be null. java.util.Map 1.2 • static <K, V> Map<K, V> of() 9 • static <K, V> Map<K, V> of(K k1, V v1) 9 . . . • static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) 9 yields an immutable map of the given keys and values, which must not be null. • static <K,V> Map.Entry<K,V> entry(K k, V v) 9 yields an immutable map entry of the given key and value, which must not be null. • static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries) 9 yields an immutable map of the given entries. From the Library of Hristo Dimov Hristov 9.5 Views and Wrappers 539 java.util.Collections 1.2 • • • • • • • • static static static static static static static static <E> <E> <E> <E> <E> <K, <K, <K, Collection unmodifiableCollection(Collection<E> c) List unmodifiableList(List<E> c) Set unmodifiableSet(Set<E> c) SortedSet unmodifiableSortedSet(SortedSet<E> c) SortedSet unmodifiableNavigableSet(NavigableSet<E> c) 8 V> Map unmodifiableMap(Map<K, V> c) V> SortedMap unmodifiableSortedMap(SortedMap<K, V> c) V> SortedMap unmodifiableNavigableMap(NavigableMap<K, V> c) 8 constructs a view of the collection; the view’s mutator methods throw an UnsupportedOperationException. • • • • • • • • static static static static static static static static <E> <E> <E> <E> <E> <K, <K, <K, Collection<E> synchronizedCollection(Collection<E> c) List synchronizedList(List<E> c) Set synchronizedSet(Set<E> c) SortedSet synchronizedSortedSet(SortedSet<E> c) NavigableSet synchronizedNavigableSet(NavigableSet<E> c) 8 V> Map<K, V> synchronizedMap(Map<K, V> c) V> SortedMap<K, V> synchronizedSortedMap(SortedMap<K, V> c) V> NavigableMap<K, V> synchronizedNavigableMap(NavigableMap<K, V> c) 8 constructs a view of the collection; the view’s methods are synchronized. • • • • • • • static <E> Collection checkedCollection(Collection<E> c, Class<E> elementType) static <E> List checkedList(List<E> c, Class<E> elementType) static <E> Set checkedSet(Set<E> c, Class<E> elementType) static <E> SortedSet checkedSortedSet(SortedSet<E> c, Class<E> elementType) static <E> NavigableSet checkedNavigableSet(NavigableSet<E> c, Class<E> elementType) 8 static <K, V> Map checkedMap(Map<K, V> c, Class<K> keyType, Class<V> valueType) static <K, V> SortedMap checkedSortedMap(SortedMap<K, V> c, Class<K> keyType, Class<V> valueType) • static <K, V> NavigableMap checkedNavigableMap(NavigableMap<K, V> c, Class<K> keyType, Class<V> valueType) 8 • static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> elementType) 8 constructs a view of the collection; the view’s methods throw a ClassCastException if an element of the wrong type is inserted. • static <E> List<E> nCopies(int n, E value) yields an unmodifiable list with n identical values. (Continues) From the Library of Hristo Dimov Hristov 540 Chapter 9 Collections java.util.Collections 1.2 (Continued) • static <E> List<E> singletonList(E value) • static <E> Set<E> singleton(E value) • static <K, V> Map<K, V> singletonMap(K key, V value) yields a singleton list, set, or map. As of Java 9, use one of the of methods instead. • • • • • • • • • • static static static static static static static static static static <E> List<E> emptyList() <T> Set<T> emptySet() <E> SortedSet<E> emptySortedSet() NavigableSet<E> emptyNavigableSet() <K,V> Map<K,V> emptyMap() <K,V> SortedMap<K,V> emptySortedMap() <K,V> NavigableMap<K,V> emptyNavigableMap() <T> Enumeration<T> emptyEnumeration() <T> Iterator<T> emptyIterator() <T> ListIterator<T> emptyListIterator() yields an empty collection, map, or iterator. java.util.Arrays 1.2 • static <E> List<E> asList(E... array) returns a list view of the elements in an array that is modifiable but not resizable. java.util.List<E> 1.2 • List<E> subList(int firstIncluded, int firstExcluded) returns a list view of the elements within a range of positions. java.util.SortedSet<E> 1.2 • SortedSet<E> subSet(E firstIncluded, E firstExcluded) • SortedSet<E> headSet(E firstExcluded) • SortedSet<E> tailSet(E firstIncluded) returns a view of the elements within a range. From the Library of Hristo Dimov Hristov 9.6 Algorithms 541 java.util.NavigableSet<E> 6 • NavigableSet<E> subSet(E from, boolean fromIncluded, E to, boolean toIncluded) • NavigableSet<E> headSet(E to, boolean toIncluded) • NavigableSet<E> tailSet(E from, boolean fromIncluded) returns a view of the elements within a range. The boolean flags determine whether the bounds are included in the view. java.util.SortedMap<K, V> 1.2 • SortedMap<K, V> subMap(K firstIncluded, K firstExcluded) • SortedMap<K, V> headMap(K firstExcluded) • SortedMap<K, V> tailMap(K firstIncluded) returns a map view of the entries whose keys are within a range. java.util.NavigableMap<K, V> 6 • NavigableMap<K, V> subMap(K from, boolean fromIncluded, K to, boolean toIncluded) • NavigableMap<K, V> headMap(K from, boolean fromIncluded) • NavigableMap<K, V> tailMap(K to, boolean toIncluded) returns a map view of the entries whose keys are within a range. The boolean flags determine whether the bounds are included in the view. 9.6 Algorithms In addition to implementing collection classes, the Java collections framework also provides a number of useful algorithms. In the following sections, you will see how to use these algorithms and how to write your own algorithms that work well with the collections framework. 9.6.1 Why Generic Algorithms? Generic collection interfaces have a great advantage—you only need to implement your algorithms once. For example, consider a simple algorithm to compute the maximum element in a collection. Traditionally, programmers would implement such an algorithm as a loop. Here is how you can find the largest element of an array. if (a.length == 0) throw new NoSuchElementException(); T largest = a[0]; From the Library of Hristo Dimov Hristov 542 Chapter 9 Collections for (int i = 1; i < a.length; i++) if (largest.compareTo(a[i]) < 0) largest = a[i]; Of course, to find the maximum of an array list, you would write the code slightly differently. if (v.size() == 0) throw new NoSuchElementException(); T largest = v.get(0); for (int i = 1; i < v.size(); i++) if (largest.compareTo(v.get(i)) < 0) largest = v.get(i); What about a linked list? You don’t have efficient random access in a linked list, but you can use an iterator. if (l.isEmpty()) throw new NoSuchElementException(); Iterator<T> iter = l.iterator(); T largest = iter.next(); while (iter.hasNext()) { T next = iter.next(); if (largest.compareTo(next) < 0) largest = next; } These loops are tedious to write, and just a bit error-prone. Is there an offby-one error? Do the loops work correctly for empty containers? For containers with only one element? You don’t want to test and debug this code every time, but you also don’t want to implement a whole slew of methods, such as these: static <T extends Comparable> T max(T[] a) static <T extends Comparable> T max(ArrayList<T> v) static <T extends Comparable> T max(LinkedList<T> l) That’s where the collection interfaces come in. Think of the minimal collection interface that you need to efficiently carry out the algorithm. Random access with get and set comes higher in the food chain than simple iteration. As you have seen in the computation of the maximum element in a linked list, random access is not required for this task. Computing the maximum can be done simply by iterating through the elements. Therefore, you can implement the max method to take any object that implements the Collection interface. public static <T extends Comparable> T max(Collection<T> c) { if (c.isEmpty()) throw new NoSuchElementException(); Iterator<T> iter = c.iterator(); T largest = iter.next(); From the Library of Hristo Dimov Hristov 9.6 Algorithms 543 while (iter.hasNext()) { T next = iter.next(); if (largest.compareTo(next) < 0) largest = next; } return largest; } Now you can compute the maximum of a linked list, an array list, or an array, with a single method. That’s a powerful concept. In fact, the standard C++ library has dozens of useful algorithms, each operating on a generic collection. The Java library is not quite so rich, but it does contain the basics: sorting, binary search, and some utility algorithms. 9.6.2 Sorting and Shuffling Computer old-timers will sometimes reminisce about how they had to use punched cards and to actually program, by hand, algorithms for sorting. Nowadays, of course, sorting algorithms are part of the standard library for most programming languages, and the Java programming language is no exception. The sort method in the Collections class sorts a collection that implements the List interface. var staff = new LinkedList<String>(); fill collection Collections.sort(staff); This method assumes that the list elements implement the Comparable interface. If you want to sort the list in some other way, you can use the sort method of the List interface and pass a Comparator object. Here is how you can sort a list of employees by salary: staff.sort(Comparator.comparingDouble(Employee::getSalary)); If you want to sort a list in descending order, use the static convenience method Comparator.reverseOrder(). It returns a comparator that returns b.compareTo(a). For example, staff.sort(Comparator.reverseOrder()) sorts the elements in the list staff in reverse order, according to the ordering given by the compareTo method of the element type. Similarly, From the Library of Hristo Dimov Hristov 544 Chapter 9 Collections staff.sort(Comparator.comparingDouble(Employee::getSalary).reversed()) sorts by descending salary. You may wonder how the sort method sorts a list. Typically, when you look at a sorting algorithm in a book on algorithms, it is presented for arrays and uses random element access. However, random access in a linked list is inefficient. You can actually sort linked lists efficiently by using a form of merge sort. However, the implementation in the Java programming language does not do that. It simply dumps all elements into an array, sorts the array, and then copies the sorted sequence back into the list. The sort algorithm used in the collections library is a bit slower than QuickSort, the traditional choice for a general-purpose sorting algorithm. However, it has one major advantage: It is stable, that is, it doesn’t switch equal elements. Why do you care about the order of equal elements? Here is a common scenario. Suppose you have an employee list that you already sorted by name. Now you sort by salary. What happens to employees with equal salary? With a stable sort, the ordering by name is preserved. In other words, the outcome is a list that is sorted first by salary, then by name. Collections need not implement all of their “optional” methods, so all methods that receive collection parameters must describe when it is safe to pass a collection to an algorithm. For example, you clearly cannot pass an unmodifiableList list to the sort algorithm. What kind of list can you pass? According to the documentation, the list must be modifiable but need not be resizable. The terms are defined as follows: • A list is modifiable if it supports the set method. • A list is resizable if it supports the add and remove operations. The Collections class has an algorithm shuffle that does the opposite of sorting—it randomly permutes the order of the elements in a list. For example: ArrayList<Card> cards = . . .; Collections.shuffle(cards); If you supply a list that does not implement the RandomAccess interface, the shuffle method copies the elements into an array, shuffles the array, and copies the shuffled elements back into the list. The program in Listing 9.7 fills an array list with 49 Integer objects containing the numbers 1 through 49. It then randomly shuffles the list and selects the first six values from the shuffled list. Finally, it sorts the selected values and prints them. From the Library of Hristo Dimov Hristov 9.6 Algorithms 545 Listing 9.7 shuffle/ShuffleTest.java 1 package shuffle; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 /** * This program demonstrates the random shuffle and sort algorithms. * @version 1.12 2018-04-10 * @author Cay Horstmann */ public class ShuffleTest { public static void main(String[] args) { var numbers = new ArrayList<Integer>(); for (int i = 1; i <= 49; i++) numbers.add(i); Collections.shuffle(numbers); List<Integer> winningCombination = numbers.subList(0, 6); Collections.sort(winningCombination); System.out.println(winningCombination); } } java.util.Collections 1.2 • static <T extends Comparable<? super T>> void sort(List<T> elements) sorts the elements in the list, using a stable sort algorithm. The algorithm is guaranteed to run in O(n log n) time, where n is the length of the list. • static void shuffle(List<?> elements) • static void shuffle(List<?> elements, Random r) randomly shuffles the elements in the list. This algorithm runs in O(n a(n)) time, where n is the length of the list and a(n) is the average time to access an element. java.util.List<E> 1.2 • default void sort(Comparator<? super T> comparator) 8 Sorts this list, using the given comparator. From the Library of Hristo Dimov Hristov 546 Chapter 9 Collections java.util.Comparator<T> 1.2 • static <T extends Comparable<? super T>> Comparator<T> reverseOrder() 8 Yields a comparator that reverses the ordering provided by the Comparable interface. • default Comparator<T> reversed() 8 Yields a comparator that reverses the ordering provided by this comparator. 9.6.3 Binary Search To find an object in an array, you normally visit all elements until you find a match. However, if the array is sorted, you can look at the middle element and check whether it is larger than the element that you are trying to find. If so, keep looking in the first half of the array; otherwise, look in the second half. That cuts the problem in half, and you keep going in the same way. For example, if the array has 1024 elements, you will locate the match (or confirm that there is none) after 10 steps, whereas a linear search would have taken you an average of 512 steps if the element is present, and 1024 steps to confirm that it is not. The binarySearch of the Collections class implements this algorithm. Note that the collection must already be sorted, or the algorithm will return the wrong answer. To find an element, supply the collection (which must implement the List interface—more on that in the note below) and the element to be located. If the collection is not sorted by the compareTo element of the Comparable interface, supply a comparator object as well. i = Collections.binarySearch(c, element); i = Collections.binarySearch(c, element, comparator); A non-negative return value from the binarySearch method denotes the index of the matching object. That is, c.get(i) is equal to element under the comparison order. If the value is negative, then there is no matching element. However, you can use the return value to compute the location where you should insert element into the collection to keep it sorted. The insertion location is insertionPoint = -i - 1; It isn’t simply -i because then the value of 0 would be ambiguous. In other words, the operation if (i < 0) c.add(-i - 1, element); adds the element in the correct place. From the Library of Hristo Dimov Hristov 9.6 Algorithms 547 To be worthwhile, binary search requires random access. If you have to iterate one by one through half of a linked list to find the middle element, you have lost all advantage of the binary search. Therefore, the binarySearch algorithm reverts to a linear search if you give it a linked list. java.util.Collections 1.2 • static <T extends Comparable<? super T>> int binarySearch(List<T> elements, T key) • static <T> int binarySearch(List<T> elements, T key, Comparator<? super T> c) searches for a key in a sorted list, using a binary search if the element type implements the RandomAccess interface, and a linear search in all other cases. The methods are guaranteed to run in O(a(n) log n) time, where n is the length of the list and a(n) is the average time to access an element. The methods return either the index of the key in the list, or a negative value i if the key is not present in the list. In that case, the key should be inserted at index -i - 1 for the list to stay sorted. 9.6.4 Simple Algorithms The Collections class contains several simple but useful algorithms. Among them is the example from the beginning of this section—finding the maximum value of a collection. Others include copying elements from one list to another, filling a container with a constant value, and reversing a list. Why supply such simple algorithms in the standard library? Surely most programmers could easily implement them with simple loops. We like the algorithms because they make life easier for the programmer reading the code. When you read a loop that was implemented by someone else, you have to decipher the original programmer’s intentions. For example, look at this loop: for (int i = 0; i < words.size(); i++) if (words.get(i).equals("C++")) words.set(i, "Java"); Now compare the loop with the call Collections.replaceAll(words, "C++", "Java"); When you see the method call, you know right away what the code does. The API notes at the end of this section describe the simple algorithms in the Collections class. The default methods Collection.removeIf and List.replaceAll that are just a bit more complex. You provide a lambda expression to test or transform elements. For From the Library of Hristo Dimov Hristov 548 Chapter 9 Collections example, here we remove all short words and change the remaining ones to lowercase: words.removeIf(w -> w.length() <= 3); words.replaceAll(String::toLowerCase); java.util.Collections 1.2 • • • • static static static static <T extends Comparable<? super T>> T min(Collection<T> <T extends Comparable<? super T>> T max(Collection<T> <T> min(Collection<T> elements, Comparator<? super T> <T> max(Collection<T> elements, Comparator<? super T> elements) elements) c) c) returns the smallest or largest element in the collection. (The parameter bounds are simplified for clarity.) • static <T> void copy(List<? super T> to, List<T> from) copies all elements from a source list to the same positions in the target list. The target list must be at least as long as the source list. • static <T> void fill(List<? super T> l, T value) sets all positions of a list to the same value. • static <T> boolean addAll(Collection<? super T> c, T... values) 5 adds all values to the given collection and returns true if the collection changed as a result. • static <T> boolean replaceAll(List<T> l, T oldValue, T newValue) 1.4 replaces all elements equal to oldValue with newValue. • static int indexOfSubList(List<?> l, List<?> s) 1.4 • static int lastIndexOfSubList(List<?> l, List<?> s) 1.4 returns the index of the first or last sublist of l equaling s, or -1 if no sublist of l equals s. For example, if l is [s, t, a, r] and s is [t, a, r], then both methods return the index 1. • static void swap(List<?> l, int i, int j) 1.4 swaps the elements at the given offsets. • static void reverse(List<?> l) reverses the order of the elements in a list. For example, reversing the list [t, a, r] yields the list [r, a, t]. This method runs in O(n) time, where n is the length of the list. (Continues) From the Library of Hristo Dimov Hristov 9.6 Algorithms 549 java.util.Collections 1.2 (Continued) • static void rotate(List<?> l, int d) 1.4 rotates the elements in the list, moving the entry with index i to position (i + d) % l.size(). For example, rotating the list [t, a, r] by 2 yields the list [a, r, t]. This method runs in O(n) time, where n is the length of the list. • static int frequency(Collection<?> c, Object o) 5 returns the count of elements in c that equal the object o. • boolean disjoint(Collection<?> c1, Collection<?> c2) 5 returns true if the collections have no elements in common. java.util.Collection<T> 1.2 • default boolean removeIf(Predicate<? super E> filter) 8 removes all matching elements. java.util.List<E> 1.2 • default void replaceAll(UnaryOperator<E> op) 8 applies the operation to all elements of this list. 9.6.5 Bulk Operations There are several operations that copy or remove elements “in bulk.” The call coll1.removeAll(coll2); removes all elements from coll1 that are present in coll2. Conversely, coll1.retainAll(coll2); removes all elements from coll1 that are not present in coll2. Here is a typical application. Suppose you want to find the intersection of two sets—the elements that two sets have in common. First, make a new set to hold the result. var result = new HashSet<String>(firstSet); From the Library of Hristo Dimov Hristov 550 Chapter 9 Collections Here, we use the fact that every collection has a constructor whose parameter is another collection that holds the initialization values. Now, use the retainAll method: result.retainAll(secondSet); It retains all elements that occur in both sets. You have formed the intersection without programming a loop. You can carry this idea further and apply a bulk operation to a view. For example, suppose you have a map that maps employee IDs to employee objects and you have a set of the IDs of all employees that are to be terminated. Map<String, Employee> staffMap = . . .; Set<String> terminatedIDs = . . .; Simply form the key set and remove all IDs of terminated employees. staffMap.keySet().removeAll(terminatedIDs); Since the key set is a view into the map, the keys and associated employee names are automatically removed from the map. By using a subrange view, you can restrict bulk operations to sublists and subsets. For example, suppose you want to add the first ten elements of a list to another container. Form a sublist to pick out the first ten: relocated.addAll(staff.subList(0, 10)); The subrange can also be a target of a mutating operation. staff.subList(0, 10).clear(); 9.6.6 Converting between Collections and Arrays Large portions of the Java platform API were designed before the collections framework was created. As a result, you will occasionally need to translate between traditional arrays and the more modern collections. If you have an array that you need to turn into a collection, the List.of wrapper serves this purpose. For example: String[] values = . . .; var staff = new HashSet<>(List.of(values)); Obtaining an array from a collection is a bit trickier. Of course, you can use the toArray method: Object[] values = staff.toArray(); From the Library of Hristo Dimov Hristov 9.6 Algorithms 551 But the result is an array of objects. Even if you know that your collection contained objects of a specific type, you cannot use a cast: String[] values = (String[]) staff.toArray(); // ERROR The array returned by the toArray method was created as an Object[] array, and you cannot change its type. Instead, use a variant of the toArray method and give it an array of length 0 of the type that you’d like. The returned array is then created as the same array type: String[] values = staff.toArray(new String[0]); If you like, you can construct the array to have the correct size: staff.toArray(new String[staff.size()]); In this case, no new array is created. NOTE: You may wonder why you can’t simply pass a Class object (such as String.class) to the toArray method. However, this method does “double duty”—both to fill an existing array (provided it is long enough) and to create a new array. 9.6.7 Writing Your Own Algorithms If you write your own algorithm (or, in fact, any method that has a collection as a parameter), you should work with interfaces, not concrete implementations, whenever possible. For example, suppose you want to process items. Of course, you can implement a method like this: public void processItems(ArrayList<Item> items) { for (Item item : items) do something with item } However, you now constrained the caller of your method—the caller must supply the items in an ArrayList. If the items happen to be in another collection, they first need to be repackaged. It is much better to accept a more general collection. You should ask yourself this: What is the most general collection interface that can do the job? Do you care about the order? Then you should accept a List. But if the order doesn’t matter, you can accept collections of any kind: From the Library of Hristo Dimov Hristov 552 Chapter 9 Collections public void processItems(Collection<Item> items) { for (Item item : items) do something with item } Now, anyone can call this method with an ArrayList or a LinkedList, or even with an array wrapped with the List.of wrapper. TIP: In this case, you can do even better by accepting an Iterable<Item>. The Iterable interface has a single abstract method iterator which the enhanced for loop uses behind the scenes. The Collection interface extends Iterable. Conversely, if your method returns multiple elements, you don’t want to constrain yourself against future improvements. For example, consider public ArrayList<Item> lookupItems(. . .) { var result = new ArrayList<Item>(); . . . return result; } This method promises to return an ArrayList, even though the caller almost certainly doesn’t care what kind of lists it is. If instead you return a List, you can at any time add a branch that returns an empty or singleton list by calling List.of. NOTE: If it is such a good idea to use collection interfaces as parameter and return type, why doesn’t the Java library follow this rule consistently? For example, the JComboBox class has two constructors: JComboBox(Object[] items) JComboBox(Vector<?> items) The reason is simply timing. The Swing library was created before the collections library. 9.7 Legacy Collections A number of “legacy” container classes have been present since the first release of Java, before there was a collections framework. From the Library of Hristo Dimov Hristov 9.7 Legacy Collections 553 They have been integrated into the collections framework—see Figure 9.12. We briefly introduce them in the following sections. Figure 9.12 Legacy classes in the collections framework 9.7.1 The Hashtable Class The classic Hashtable class serves the same purpose as the HashMap class and has essentially the same interface. Just like methods of the Vector class, the Hashtable methods are synchronized. If you do not require compatibility with legacy code, you should use a HashMap instead. If you need concurrent access, use a ConcurrentHashMap—see Chapter 12. 9.7.2 Enumerations The legacy collections use the Enumeration interface for traversing sequences of elements. The Enumeration interface has two methods, hasMoreElements and nextElement. These are entirely analogous to the hasNext and next methods of the Iterator interface. From the Library of Hristo Dimov Hristov 554 Chapter 9 Collections If you find this interface with legacy classes, you can use Collections.list to collect the elements in an ArrayList. For example, the LogManager class is only willing to reveal logger names as an Enumeration. Here is how you can get them all: ArrayList<String> loggerNames = Collections.list(LogManager.getLoggerNames()); Alternatively, as of Java 9, you can turn an enumeration into an iterator: LogManager.getLoggerNames().asIterator().forEachRemaining(n -> { . . . }); You will occasionally encounter a legacy method that expects an enumeration parameter. The static method Collections.enumeration yields an enumeration object that enumerates the elements in the collection. For example: List<InputStream> streams = . . .; var in = new SequenceInputStream(Collections.enumeration(streams)); // the SequenceInputStream constructor expects an enumeration NOTE: In C++, it is quite common to use iterators as parameters. Fortunately, on the Java platform, very few programmers use this idiom. It is much smarter to pass around the collection than to pass an iterator. The collection object is more useful. The recipients can always obtain the iterator from the collection when they need to do so, plus they have all the collection methods at their disposal. However, you will find enumerations in some legacy code because they were the only available mechanism for generic collections until the collections framework appeared in Java 1.2. java.util.Enumeration<E> 1.0 • boolean hasMoreElements() returns true if there are more elements yet to be inspected. • E nextElement() returns the next element to be inspected. Do not call this method if hasMoreElements() returned false. • default Iterator<E> asIterator() 9 yields an iterator that iterates over the enumerated elements. From the Library of Hristo Dimov Hristov 9.7 Legacy Collections 555 java.util.Collections 1.2 • static <T> Enumeration<T> enumeration(Collection<T> c) returns an enumeration that enumerates the elements of c. • public static <T> ArrayList<T> list(Enumeration<T> e) returns an array list containing the elements enumerated by e. 9.7.3 Property Maps A property map is a map structure of a special type. It has three particular characteristics: • The keys and values are strings. • The map can easily be saved to a file and loaded from a file. • There is a secondary table for default values. The Java platform class that implements a property map is called Properties. Property maps are useful in specifying configuration options for programs. For example: var settings = new Properties(); settings.setProperty("width", "600.0"); settings.setProperty("filename", "/home/cay/books/cj11/code/v1ch11/raven.html"); Use the store method to save map list of properties to a file. Here, we just save the property map in the file program.properties. The second argument is a comment that is included in the file. var out = new FileOutputStream("program.properties"); settings.store(out, "Program Properties"); The sample set gives the following output: #Program Properties #Sun Dec 31 12:54:19 PST 2017 top=227.0 left=1286.0 width=423.0 height=547.0 filename=/home/cay/books/cj11/code/v1ch11/raven.html To load the properties from a file, use var in = new FileInputStream("program.properties"); settings.load(in); From the Library of Hristo Dimov Hristov 556 Chapter 9 Collections The System.getProperties method yields a Properties object to describe system information. For example, the home directory has the key "user.home". You can read it with the getProperties method that yields the key as a string: String userDir = System.getProperty("user.home"); CAUTION: For historical reasons, the Properties class implements Map<Object, Object>. Therefore, you can use the get and put methods of the Map interface. But the get method returns the type Object, and the put method allows you to insert any object. It is best to stick with the getProperty and setProperty methods that work with strings, not objects. To get the Java version of the virtual machine, look up the "java.version" property. You get a string such as "11.0.1" (or "1.8.0" for Java 8.) TIP: As you can see, the version numbering changed in Java 9. This seemingly small change broke a good number of tools that had relied on the old format. If you parse the version string, be sure to read JEP 322 at http://openjdk.java.net /jeps/322 to see how version strings will be formatted in the future—or at least, until the numbering scheme changes again. The Properties class has two mechanisms for providing defaults. First, whenever you look up the value of a string, you can specify a default that should be used automatically when the key is not present. String filename = settings.getProperty("filename", ""); If there is a "filename" property in the property map, filename is set to that string. Otherwise, filename is set to the empty string. If you find it too tedious to specify the default in every call to getProperty, you can pack all the defaults into a secondary property map and supply that map in the constructor of your primary property map. var defaultSettings = new Properties(); defaultSettings.setProperty("width", "600"); defaultSettings.setProperty("height", "400"); defaultSettings.setProperty("filename", ""); . . . var settings = new Properties(defaultSettings); Yes, you can even specify defaults to defaults if you give another property map parameter to the defaultSettings constructor, but it is not something one would normally do. From the Library of Hristo Dimov Hristov 9.7 Legacy Collections 557 The companion code has a sample program that shows how you can use properties for storing and loading program state. The program uses the ImageViewer program from Chapter 2 and remembers the frame position, size, and last loaded file. Run the program, load a file, and move and resize the window. Then close the program and reopen it to see that it remembers your file and your favorite window placement. You can also manually edit the file .corejava/ImageViewer.properties in your home directory. NOTE: Prior to Java 9, properties files used the 7-bit ASCII encoding. Nowadays, they use UTF-8. Properties are simple tables without a hierarchical structure. It is common to introduce a fake hierarchy with key names such as window.main.color, window.main.title, and so on. But the Properties class has no methods that help organize such a hierarchy. If you store complex configuration information, you should use the Preferences class instead—see Chapter 10. java.util.Properties 1.0 • Properties() creates an empty property map. • Properties(Properties defaults) creates an empty property map with a set of defaults. • String getProperty(String key) gets a property. Returns the string associated with the key, or the string associated with the key in the default table if it wasn’t present in the table, or null if the key wasn’t present in the default table either. • String getProperty(String key, String defaultValue) gets a property with a default value if the key is not found. Returns the string associated with the key, or the default string if it wasn’t present in the table. • Object setProperty(String key, String value) sets a property. Returns the previously set value of the given key. • void load(InputStream in) throws IOException loads a property map from an input stream. • void store(OutputStream out, String header) 1.2 saves a property map to an output stream. The header in the first line of the stored file. From the Library of Hristo Dimov Hristov 558 Chapter 9 Collections java.lang.System 1.0 • Properties getProperties() retrieves all system properties. The application must have permission to retrieve all properties, or a security exception is thrown. • String getProperty(String key) retrieves the system property with the given key name. The application must have permission to retrieve the property, or a security exception is thrown. The following properties can always be retrieved: java.version java.vendor java.vendor.url java.home java.class.path java.library.path java.class.version os.name os.version os.arch file.separator path.separator line.separator java.io.tempdir user.name user.home user.dir java.compiler java.specification.version java.specification.vendor java.specification.name java.vm.specification.version java.vm.specification.vendor java.vm.specification.name java.vm.version java.vm.vendor java.vm.name 9.7.4 Stacks Since version 1.0, the standard library had a Stack class with the familiar push and pop methods. However, the Stack class extends the Vector class, which is not satisfactory from a theoretical perspective—you can apply such un-stacklike operations as insert and remove to insert and remove values anywhere, not just at the top of the stack. From the Library of Hristo Dimov Hristov 9.7 Legacy Collections 559 java.util.Stack<E> 1.0 • E push(E item) pushes item onto the stack and returns item. • E pop() pops and returns the top item of the stack. Don’t call this method if the stack is empty. • E peek() returns the top of the stack without popping it. Don’t call this method if the stack is empty. 9.7.5 Bit Sets The Java platform’s BitSet class stores a sequence of bits. (It is not a set in the mathematical sense—bit vector or bit array would have been more appropriate terms.) Use a bit set if you need to store a sequence of bits (for example, flags) efficiently. A bit set packs the bits into bytes, so it is far more efficient to use a bit set than an ArrayList of Boolean objects. The BitSet class gives you a convenient interface for reading, setting, and resetting individual bits. Using this interface avoids the masking and other bitfiddling operations that are necessary if you store bits in int or long variables. For example, for a BitSet named bucketOfBits, bucketOfBits.get(i) returns true if the ith bit is on, and false otherwise. Similarly, bucketOfBits.set(i) turns the ith bit on. Finally, bucketOfBits.clear(i) turns the ith bit off. C++ NOTE: The C++ bitset template has the same functionality as the Java platform BitSet. From the Library of Hristo Dimov Hristov 560 Chapter 9 Collections java.util.BitSet 1.0 • BitSet(int initialCapacity) constructs a bit set. • int length() returns the “logical length” of the bit set: 1 plus the index of the highest set bit. • boolean get(int bit) gets a bit. • void set(int bit) sets a bit. • void clear(int bit) clears a bit. • void and(BitSet set) logically ANDs this bit set with another. • void or(BitSet set) logically ORs this bit set with another. • void xor(BitSet set) logically XORs this bit set with another. • void andNot(BitSet set) clears all bits in this bit set that are set in the other bit set. As an example of using bit sets, we want to show you an implementation of the “sieve of Eratosthenes” algorithm for finding prime numbers. (A prime number is a number like 2, 3, or 5 that is divisible only by itself and 1, and the sieve of Eratosthenes was one of the first methods discovered to enumerate these fundamental building blocks.) This isn’t a terribly good algorithm for finding the primes, but for some reason it has become a popular benchmark for compiler performance. (It isn’t a good benchmark either, because it mainly tests bit operations.) Oh well, we bow to tradition and present an implementation. This program counts all prime numbers between 2 and 2,000,000. (There are 148,933 primes in this interval, so you probably don’t want to print them all out.) Without going into too many details of this program, the idea is to march through a bit set with 2 million bits. First, we turn on all the bits. After that, we turn off the bits that are multiples of numbers known to be prime. The positions of the bits that remain after this process are themselves prime From the Library of Hristo Dimov Hristov 9.7 Legacy Collections 561 numbers. Listing 9.8 lists this program in the Java programming language, and Listing 9.9 is the C++ code. NOTE: Even though the sieve isn’t a good benchmark, we couldn’t resist timing the two implementations of the algorithm. Here are the timing results with a i7-8550U processor and 16 GB of RAM, running Ubuntu 17.10: • C++ (g++ 7.2.0): 173 milliseconds • Java (Java 11.0.1): 41 milliseconds We have run this test for ten editions of Core Java, and in the last six editions, Java easily beat C++. In all fairness, if one cranks up the optimization level in the C++ compiler, it beats Java with a time of 34 milliseconds. Java could only match that if the program ran long enough to trigger the Hotspot just-in-time compiler. Listing 9.8 sieve/Sieve.java 1 package sieve; 2 3 import java.util.*; 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 /** * This program runs the Sieve of Erathostenes benchmark. It computes all primes * up to 2,000,000. * @version 1.21 2004-08-03 * @author Cay Horstmann */ public class Sieve { public static void main(String[] s) { int n = 2000000; long start = System.currentTimeMillis(); var bitSet = new BitSet(n + 1); int count = 0; int i; for (i = 2; i <= n; i++) bitSet.set(i); i = 2; while (i * i <= n) { if (bitSet.get(i)) { (Continues) From the Library of Hristo Dimov Hristov 562 Chapter 9 Collections Listing 9.8 (Continued) count++; int k = 2 * i; while (k <= n) { bitSet.clear(k); k += i; } 27 28 29 30 31 32 33 } i++; 34 35 } while (i <= n) { if (bitSet.get(i)) count++; i++; } long end = System.currentTimeMillis(); System.out.println(count + " primes"); System.out.println((end - start) + " milliseconds"); 36 37 38 39 40 41 42 43 44 } 45 46 } Listing 9.9 sieve/sieve.cpp 1 /** @version 1.21 2004-08-03 @author Cay Horstmann 2 3 4 */ 5 6 7 8 #include <bitset> #include <iostream> #include <ctime> 9 10 using namespace std; 11 12 13 14 15 int main() { const int N = 2000000; clock_t cstart = clock(); 16 17 18 19 20 21 22 bitset<N + 1> b; int count = 0; int i; for (i = 2; i <= N; i++) b.set(i); i = 2; From the Library of Hristo Dimov Hristov 9.7 Legacy Collections 563 while (i * i <= N) { if (b.test(i)) { count++; int k = 2 * i; while (k <= N) { b.reset(k); k += i; } } i++; } while (i <= N) { if (b.test(i)) count++; i++; } 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 clock_t cend = clock(); double millis = 1000.0 * (cend - cstart) / CLOCKS_PER_SEC; 44 45 46 cout << count << " primes\n" << millis << " milliseconds\n"; 47 48 return 0; 49 50 } This completes our tour through the Java collections framework. As you have seen, the Java library offers a wide variety of collection classes for your programming needs. In the next chapter, you will learn how to write graphical user interfaces. From the Library of Hristo Dimov Hristov This page intentionally left blank From the Library of Hristo Dimov Hristov CHAPTER 10 Graphical User Interface Programming In this chapter • 10.1 A History of Java User Interface Toolkits, page 565 • 10.2 Displaying Frames, page 567 • 10.3 Displaying Information in a Component, page 574 • 10.4 Event Handling, page 598 • 10.5 The Preferences API, page 624 Java was born at a time when most computer users interacted with graphical desktop applications. Nowadays, browser-based and mobile applications are far more common, but there are still times when it is useful to provide a desktop application. In this and the following chapter, we discuss the basics of user interface programming with the Swing toolkit. If, on the other hand, you intend to use Java for server-side programming only and are not interested in writing GUI programs, you can safely skip these two chapters. 10.1 A History of Java User Interface Toolkits When Java 1.0 was introduced, it contained a class library, called the Abstract Window Toolkit (AWT), for basic GUI programming. The basic AWT library 565 From the Library of Hristo Dimov Hristov 566 Chapter 10 Graphical User Interface Programming deals with user interface elements by delegating their creation and behavior to the native GUI toolkit on each target platform (Windows, Linux, Macintosh, and so on). For example, if you used the original AWT to put a text box on a Java window, an underlying “peer” text box actually handled the text input. The resulting program could then, in theory, run on any of these platforms, with the “look-and-feel” of the target platform. The peer-based approach worked well for simple applications, but it soon became apparent that it was fiendishly difficult to write a high-quality portable graphics library depending on native user interface elements. User interface elements such as menus, scrollbars, and text fields can have subtle differences in behavior on different platforms. It was hard, therefore, to give users a consistent and predictable experience with this approach. Moreover, some graphical environments (such as X11/Motif) do not have as rich a collection of user interface components as does Windows or the Macintosh. This further limits a portable library based on a “lowest common denominator” approach. As a result, GUI applications built with the AWT simply did not look as nice as native Windows or Macintosh applications, nor did they have the kind of functionality that users of those platforms had come to expect. More depressingly, there were different bugs in the AWT user interface library on the different platforms. Developers complained that they had to test their applications on each platform—a practice derisively called “write once, debug everywhere.” In 1996, Netscape created a GUI library they called the IFC (Internet Foundation Classes) that used an entirely different approach. User interface elements, such as buttons, menus, and so on, were painted onto blank windows. The only functionality required from the underlying windowing system was a way to put up a window and to paint on it. Thus, Netscape’s IFC widgets looked and behaved the same no matter which platform the program ran on. Sun Microsystems worked with Netscape to perfect this approach, creating a user interface library with the code name “Swing.” Swing was available as an extension to Java 1.1 and became a part of the standard library in Java 1.2. Swing is now the official name for the non-peer-based GUI toolkit. NOTE: Swing is not a complete replacement for the AWT—it is built on top of the AWT architecture. Swing simply gives you more capable user interface components. Whenever you write a Swing program, you use the foundations of the AWT—in particular, event handling. From now on, we say “Swing” when we mean the “painted” user interface classes, and we say “AWT” when we mean the underlying mechanisms of the windowing toolkit, such as event handling. From the Library of Hristo Dimov Hristov 10.2 Displaying Frames 567 Swing has to work hard painting every pixel of the user interface. When Swing was first released, users complained that it was slow. (You can still get a feel for the problem if you run Swing applications on hardware such as a Raspberry Pi.) After a while, desktop computers got faster, and users complained that Swing was ugly—indeed, it had fallen behind the native widgets that had been spruced up with animations and fancy effects. More ominously, Adobe Flash was increasingly used to create user interfaces with even flashier effects that didn’t use the native controls at all. In 2007, Sun Microsystems introduced an entirely different user interface toolkit, called JavaFX, as a competitor to Flash. It ran on the Java VM but had its own programming language, called JavaFX Script. The language was optimized for programming animations and fancy effects. Programmers complained about the need to learn a new language, and they stayed away in droves. In 2011, Oracle released a new version, JavaFX 2.0, that had a Java API and no longer needed a separate programming language. Starting with Java 7 update 6, JavaFX has been bundled with the JDK and JRE. However, as this book is being written, Oracle has declared that JavaFX will no longer be bundled with Java, starting with version 11. Since this is a book about the core Java language and APIs, we will focus on Swing for user interface programming. TIP: We provide you with a bonus chapter that introduces JavaFX. If you have a printed copy of this book, download a free PDF from http://horstmann.com /corejava. The ebook has the chapter at the end. 10.2 Displaying Frames A top-level window (that is, a window that is not contained inside another window) is called a frame in Java. The AWT library has a class, called Frame, for this top level. The Swing version of this class is called JFrame and extends the Frame class. The JFrame is one of the few Swing components that is not painted on a canvas. Thus, the decorations (buttons, title bar, icons, and so on) are drawn by the user’s windowing system, not by Swing. CAUTION: Most Swing component classes start with a “J”: JButton, JFrame, and so on. There are classes such as Button and Frame, but they are AWT components. If you accidentally omit a “J”, your program may still compile and run, but the mixture of Swing and AWT components can lead to visual and behavioral inconsistencies. From the Library of Hristo Dimov Hristov 568 Chapter 10 Graphical User Interface Programming 10.2.1 Creating a Frame In this section, we will go over the most common methods for working with a Swing JFrame. Listing 10.1 lists a simple program that displays an empty frame on the screen, as illustrated in Figure 10.1. Figure 10.1 The simplest visible frame Listing 10.1 simpleframe/SimpleFrameTest.java 1 package simpleFrame; 2 3 4 import java.awt.*; import javax.swing.*; 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 /** * @version 1.34 2018-04-10 * @author Cay Horstmann */ public class SimpleFrameTest { public static void main(String[] args) { EventQueue.invokeLater(() -> { var frame = new SimpleFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }); } } 22 From the Library of Hristo Dimov Hristov 10.2 23 24 25 26 Displaying Frames 569 class SimpleFrame extends JFrame { private static final int DEFAULT_WIDTH = 300; private static final int DEFAULT_HEIGHT = 200; 27 public SimpleFrame() { setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT); } 28 29 30 31 32 } Let’s work through this program, line by line. The Swing classes are placed in the javax.swing package. The package name indicates a Java extension package, not a core package. For historical reasons, Swing is considered an extension. However, it is present in every Java implementation since version 1.2. javax By default, a frame has a rather useless size of 0 × 0 pixels. We define a subclass SimpleFrame whose constructor sets the size to 300 × 200 pixels. This is the only difference between a SimpleFrame and a JFrame. In the main method of the SimpleFrameTest class, we construct a SimpleFrame object and make it visible. There are two technical issues that we need to address in every Swing program. First, all Swing components must be configured from the event dispatch thread, the thread of control that passes events such as mouse clicks and keystrokes to the user interface components. The following code fragment is used to execute statements in the event dispatch thread: EventQueue.invokeLater(() -> { statements }); NOTE: You will see many Swing programs that do not initialize the user interface in the event dispatch thread. It used to be perfectly acceptable to carry out the initialization in the main thread. Sadly, as Swing components got more complex, the developers of the JDK were no longer able to guarantee the safety of that approach. The probability of an error is extremely low, but you would not want to be one of the unlucky few who encounter an intermittent problem. It is better to do the right thing, even if the code looks rather mysterious. From the Library of Hristo Dimov Hristov 570 Chapter 10 Graphical User Interface Programming Next, we define what should happen when the user closes the application’s frame. For this particular program, we want the program to exit. To select this behavior, we use the statement frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); In other programs with multiple frames, you would not want the program to exit just because the user closed one of the frames. By default, a frame is hidden when the user closes it, but the program does not terminate. (It might have been nice if the program terminated once the last frame becomes invisible, but that is not how Swing works.) Simply constructing a frame does not automatically display it. Frames start their life invisible. That gives the programmer the chance to add components into the frame before showing it for the first time. To show the frame, the main method calls the setVisible method of the frame. After scheduling the initialization statements, the main method exits. Note that exiting main does not terminate the program—just the main thread. The event dispatch thread keeps the program alive until it is terminated, either by closing the frame or by calling the System.exit method. The running program is shown in Figure 10.1—it is a truly boring top-level window. As you can see in the figure, the title bar and the surrounding decorations, such as resize corners, are drawn by the operating system and not the Swing library. The Swing library draws everything inside the frame. In this program, it just fills the frame with a default background color. 10.2.2 Frame Properties The JFrame class itself has only a few methods for changing how frames look. Of course, through the magic of inheritance, most of the methods for working with the size and position of a frame come from the various superclasses of JFrame. Here are some of the most important methods: • The setLocation and setBounds methods for setting the position of the frame • The setIconImage method, which tells the windowing system which icon to display in the title bar, task switcher window, and so on • The setTitle method for changing the text in the title bar • The setResizable method, which takes a boolean to determine if a frame will be resizeable by the user Figure 10.2 illustrates the inheritance hierarchy for the JFrame class. As the API notes indicate, the Component class (which is the ancestor of all GUI objects) and the Window class (which is the superclass of the Frame class) are From the Library of Hristo Dimov Hristov 10.2 Displaying Frames 571 Figure 10.2 Inheritance hierarchy for the frame and component classes in AWT and Swing where you need to look for the methods to resize and reshape frames. For example, the setLocation method in the Component class is one way to reposition a component. If you make the call From the Library of Hristo Dimov Hristov 572 Chapter 10 Graphical User Interface Programming setLocation(x, y) the top left corner is located x pixels across and y pixels down, where (0, 0) is the top left corner of the screen. Similarly, the setBounds method in Component lets you resize and relocate a component (in particular, a JFrame) in one step, as setBounds(x, y, width, height) Many methods of component classes come in getter/setter pairs, such as the following methods of the Frame class: public String getTitle() public void setTitle(String title) Such a getter/setter pair is called a property. A property has a name and a type. The name is obtained by changing the first letter after the get or set to lowercase. For example, the Frame class has a property with name title and type String. Conceptually, title is a property of the frame. When we set the property, we expect the title to change on the user’s screen. When we get the property, we expect to get back the value that we have set. There is one exception to the get/set convention: For properties of type boolean, the getter starts with is. For example, the following two methods define the resizable property: public boolean isResizable() public void setResizable(boolean resizable) To determine an appropriate size for a frame, first find out the screen size. Call the static getDefaultToolkit method of the Toolkit class to get the Toolkit object. (The Toolkit class is a dumping ground for a variety of methods interfacing with the native windowing system.) Then call the getScreenSize method, which returns the screen size as a Dimension object. A Dimension object simultaneously stores a width and a height, in public (!) instance variables width and height. Then you can use a suitable percentage of the screen size to size the frame. Here is the code: Toolkit kit = Toolkit.getDefaultToolkit(); Dimension screenSize = kit.getScreenSize(); int screenWidth = screenSize.width; int screenHeight = screenSize.height; setSize(screenWidth / 2, screenHeight / 2); You can also supply frame icon: Image img = new ImageIcon("icon.gif").getImage(); setIconImage(img); From the Library of Hristo Dimov Hristov 10.2 Displaying Frames 573 java.awt.Component 1.0 • boolean isVisible() • void setVisible(boolean b) gets or sets the visible property. Components are initially visible, with the exception of top-level components such as JFrame. • void setSize(int width, int height) 1.1 resizes the component to the specified width and height. • void setLocation(int x, int y) 1.1 moves the component to a new location. The x and y coordinates use the coordinates of the container if the component is not a top-level component, or the coordinates of the screen if the component is top level (for example, a JFrame). • void setBounds(int x, int y, int width, int height) 1.1 moves and resizes this component. • Dimension getSize() 1.1 • void setSize(Dimension d) 1.1 gets or sets the size property of this component. java.awt.Window 1.0 • void setLocationByPlatform(boolean b) 5 gets or sets the locationByPlatform property. When the property is set before this window is displayed, the platform picks a suitable location. java.awt.Frame 1.0 • boolean isResizable() • void setResizable(boolean b) gets or sets the resizable property. When the property is set, the user can resize the frame. • String getTitle() • void setTitle(String s) gets or sets the title property that determines the text in the title bar for the frame. (Continues) From the Library of Hristo Dimov Hristov 574 Chapter 10 Graphical User Interface Programming java.awt.Frame 1.0 (Continued) • Image getIconImage() • void setIconImage(Image image) gets or sets the iconImage property that determines the icon for the frame. The windowing system may display the icon as part of the frame decoration or in other locations. java.awt.Toolkit 1.0 • static Toolkit getDefaultToolkit() returns the default toolkit. • Dimension getScreenSize() gets the size of the user’s screen. javax.swing.ImageIcon 1.2 • ImageIcon(String filename) constructs an icon whose image is stored in a file. • Image getImage() gets the image of this icon. 10.3 Displaying Information in a Component In this section, we will show you how to display information inside a frame (Figure 10.3). You could draw the message string directly onto a frame, but that is not considered good programming practice. In Java, frames are really designed to be containers for components, such as a menu bar and other user interface elements. You normally draw on another component which you add to the frame. The structure of a JFrame is surprisingly complex. Look at Figure 10.4 which shows the makeup of a JFrame. As you can see, four panes are layered in a JFrame. The root pane, layered pane, and glass pane are of no interest to us; they are required to organize the menu bar and content pane and to implement the look-and-feel. The part that most concerns Swing programmers is From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 575 Figure 10.3 A frame that displays information the content pane. Any components that you add to a frame are automatically placed into the content pane: Component c = . . .; frame.add(c); // added to the content pane Figure 10.4 Internal structure of a JFrame From the Library of Hristo Dimov Hristov 576 Chapter 10 Graphical User Interface Programming In our case, we want to add a single component to the frame onto which we will draw our message. To draw on a component, you define a class that extends JComponent and override the paintComponent method in that class. The paintComponent method takes one parameter of type Graphics. A Graphics object remembers a collection of settings for drawing images and text, such as the font you set or the current color. All drawing in Java must go through a Graphics object. It has methods that draw patterns, images, and text. Here’s how to make a component onto which you can draw: class MyComponent extends JComponent { public void paintComponent(Graphics g) { code for drawing } } Each time a window needs to be redrawn, no matter what the reason, the event handler notifies the component. This causes the paintComponent methods of all components to be executed. Never call the paintComponent method yourself. It is called automatically whenever a part of your application needs to be redrawn, and you should not interfere with this automatic process. What sorts of actions trigger this automatic response? For example, painting occurs when the user increases the size of the window, or minimizes and then restores the window. If the user popped up another window that covered an existing window and then made the overlaid window disappear, the window that was covered is now corrupted and will need to be repainted. (The graphics system does not save the pixels underneath.) And, of course, when the window is displayed for the first time, it needs to process the code that specifies how and where it should draw the initial elements. TIP: If you need to force repainting of the screen, call the repaint method instead of paintComponent. The repaint method will cause paintComponent to be called for all components, with a properly configured Graphics object. As you saw in the code fragment above, the paintComponent method takes a single parameter of type Graphics. Measurement on a Graphics object for screen From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 577 display is done in pixels. The (0, 0) coordinate denotes the top left corner of the component on whose surface you are drawing. The Graphics class has various drawing methods, and displaying text is considered a special kind of drawing. Our paintComponent method looks like this: public class NotHelloWorldComponent extends JComponent { public static final int MESSAGE_X = 75; public static final int MESSAGE_Y = 100; public void paintComponent(Graphics g) { g.drawString("Not a Hello, World program", MESSAGE_X, MESSAGE_Y); } . . . } Finally, a component should tell its users how big it would like to be. Override the getPreferredSize method and return an object of the Dimension class with the preferred width and height: public class NotHelloWorldComponent extends JComponent { private static final int DEFAULT_WIDTH = 300; private static final int DEFAULT_HEIGHT = 200; . . . public Dimension getPreferredSize() { return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT); } } When you fill a frame with one or more components, and you simply want to use their preferred size, call the pack method instead of the setSize method: class NotHelloWorldFrame extends JFrame { public NotHelloWorldFrame() { add(new NotHelloWorldComponent()); pack(); } } Listing 10.2 shows the complete code. From the Library of Hristo Dimov Hristov 578 Chapter 10 Graphical User Interface Programming Listing 10.2 notHelloWorld/NotHelloWorld.java 1 package notHelloWorld; 2 3 4 import javax.swing.*; import java.awt.*; 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 /** * @version 1.34 2018-04-10 * @author Cay Horstmann */ public class NotHelloWorld { public static void main(String[] args) { EventQueue.invokeLater(() -> { var frame = new NotHelloWorldFrame(); frame.setTitle("NotHelloWorld"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }); } } 23 24 25 26 27 28 29 30 31 32 33 34 /** * A frame that contains a message panel. */ class NotHelloWorldFrame extends JFrame { public NotHelloWorldFrame() { add(new NotHelloWorldComponent()); pack(); } } 35 36 37 38 39 40 41 42 /** * A component that displays a message. */ class NotHelloWorldComponent extends JComponent { public static final int MESSAGE_X = 75; public static final int MESSAGE_Y = 100; 43 44 45 private static final int DEFAULT_WIDTH = 300; private static final int DEFAULT_HEIGHT = 200; 46 From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 579 public void paintComponent(Graphics g) { g.drawString("Not a Hello, World program", MESSAGE_X, MESSAGE_Y); } 47 48 49 50 51 public Dimension getPreferredSize() { return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT); } 52 53 54 55 56 } javax.swing.JFrame 1.2 • Component add(Component c) adds and returns the given component to the content pane of this frame. java.awt.Component 1.0 • void repaint() causes a repaint of the component “as soon as possible.” • Dimension getPreferredSize() is the method to override to return the preferred size of this component. javax.swing.JComponent 1.2 • void paintComponent(Graphics g) is the method to override to describe how your component needs to be painted. java.awt.Window 1.0 • void pack() resizes this window, taking into account the preferred sizes of its components. 10.3.1 Working with 2D Shapes Starting with Java 1.0, the Graphics class has methods to draw lines, rectangles, ellipses, and so on. But those drawing operations are very limited. We will instead use the shape classes from the Java 2D library. To use this library, you need to obtain an object of the Graphics2D class. This class is a subclass of the Graphics class. Ever since Java 1.2, methods such as From the Library of Hristo Dimov Hristov 580 Chapter 10 Graphical User Interface Programming automatically receive an object of the Graphics2D class. Simply use a cast, as follows: paintComponent public void paintComponent(Graphics g) { Graphics2D g2 = (Graphics2D) g; . . . } The Java 2D library organizes geometric shapes in an object-oriented fashion. In particular, there are classes to represent lines, rectangles, and ellipses: Line2D Rectangle2D Ellipse2D These classes all implement the Shape interface. The Java 2D library supports more complex shapes—arcs, quadratic and cubic curves, and general paths—that we do not discuss in this chapter. To draw a shape, you first create an object of a class that implements the Shape interface and then call the draw method of the Graphics2D class. For example: Rectangle2D rect = . . .; g2.draw(rect); The Java 2D library uses floating-point coordinates, not integers, for pixels. Internal calculations are carried out with single-precision float quantities. Single precision is sufficient—after all, the ultimate purpose of the geometric computations is to set pixels on the screen or printer. As long as any roundoff errors stay within one pixel, the visual outcome is not affected. However, manipulating float values is sometimes inconvenient for the programmer because Java is adamant about requiring casts when converting double values into float values. For example, consider the following statement: float f = 1.2; // ERROR--possible loss of precision This statement does not compile because the constant 1.2 has type double, and the compiler is nervous about loss of precision. The remedy is to add an F suffix to the floating-point constant: float f = 1.2F; // OK Now consider this statement: float f = r.getWidth(); // ERROR This statement does not compile either, for the same reason. The getWidth method returns a double. This time, the remedy is to provide a cast: From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 581 float f = (float) r.getWidth(); // OK These suffixes and casts are a bit of a pain, so the designers of the 2D library decided to supply two versions of each shape class: one with float coordinates for frugal programmers, and one with double coordinates for the lazy ones. (In this book, we fall into the second camp and use double coordinates whenever we can.) The library designers chose a curious mechanism for packaging these choices. Consider the Rectangle2D class. This is an abstract class with two concrete subclasses, which are also static inner classes: Rectangle2D.Float Rectangle2D.Double Figure 10.5 shows the inheritance diagram. Figure 10.5 2D rectangle classes It is best to ignore the fact that the two concrete classes are static inner classes—that is just a gimmick to avoid names such as FloatRectangle2D and DoubleRectangle2D. When you construct a Rectangle2D.Float object, you supply the coordinates as float numbers. For a Rectangle2D.Double object, you supply them as double numbers. var floatRect = new Rectangle2D.Float(10.0F, 25.0F, 22.5F, 20.0F); var doubleRect = new Rectangle2D.Double(10.0, 25.0, 22.5, 20.0); The construction parameters denote the top left corner, width, and height of the rectangle. From the Library of Hristo Dimov Hristov 582 Chapter 10 Graphical User Interface Programming The Rectangle2D methods use double parameters and return values. For example, the getWidth method returns a double value, even if the width is stored as a float in a Rectangle2D.Float object. TIP: Simply use the Double shape classes to avoid dealing with float values altogether. However, if you are constructing thousands of shape objects, consider using the Float classes to conserve memory. What we just discussed for the Rectangle2D classes holds for the other shape classes as well. Furthermore, there is a Point2D class with subclasses Point2D.Float and Point2D.Double. Here is how to make a point object: var p = new Point2D.Double(10, 20); The classes Rectangle2D and Ellipse2D both inherit from the common superclass RectangularShape. Admittedly, ellipses are not rectangular, but they have a bounding rectangle (see Figure 10.6). Figure 10.6 The bounding rectangle of an ellipse The RectangularShape class defines over 20 methods that are common to these shapes, among them such useful methods as getWidth, getHeight, getCenterX, and getCenterY (but, sadly, at the time of this writing, not a getCenter method that would return the center as a Point2D object). Finally, a couple of legacy classes from Java 1.0 have been fitted into the shape class hierarchy. The Rectangle and Point classes, which store a rectangle and a point with integer coordinates, extend the Rectangle2D and Point2D classes. Figure 10.7 shows the relationships between the shape classes. However, the and Float subclasses are omitted. Legacy classes are marked with a gray fill. Double From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 583 Figure 10.7 Relationships between the shape classes Rectangle2D and Ellipse2D objects are simple to construct. You need to specify • The x and y coordinates of the top left corner; and • The width and height. For ellipses, these refer to the bounding rectangle. For example, var e = new Ellipse2D.Double(150, 200, 100, 50); constructs an ellipse that is bounded by a rectangle with the top left corner at (150, 200), width of 100, and height of 50. When constructing an ellipse, you usually know the center, width, and height, but not the corner points of the bounding rectangle (which don’t even lie on the ellipse). The setFrameFromCenter method uses the center point, but it still requires one of the four corner points. Thus, you will usually end up constructing an ellipse as follows: From the Library of Hristo Dimov Hristov 584 Chapter 10 Graphical User Interface Programming var ellipse = new Ellipse2D.Double(centerX - width / 2, centerY - height / 2, width, height); To construct a line, you supply the start and end points, either as Point2D objects or as pairs of numbers: var line = new Line2D.Double(start, end); or var line = new Line2D.Double(startX, startY, endX, endY); The program in Listing 10.3 draws a rectangle, the ellipse that is enclosed in the rectangle, a diagonal of the rectangle, and a circle that has the same center as the rectangle. Figure 10.8 shows the result. Figure 10.8 Drawing geometric shapes Listing 10.3 draw/DrawTest.java 1 package draw; 2 3 4 5 import java.awt.*; import java.awt.geom.*; import javax.swing.*; 6 From the Library of Hristo Dimov Hristov 10.3 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Displaying Information in a Component 585 /** * @version 1.34 2018-04-10 * @author Cay Horstmann */ public class DrawTest { public static void main(String[] args) { EventQueue.invokeLater(() -> { var frame = new DrawFrame(); frame.setTitle("DrawTest"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }); } } 24 25 26 27 28 29 30 31 32 33 34 35 /** * A frame that contains a panel with drawings. */ class DrawFrame extends JFrame { public DrawFrame() { add(new DrawComponent()); pack(); } } 36 37 38 39 40 41 42 43 /** * A component that displays rectangles and ellipses. */ class DrawComponent extends JComponent { private static final int DEFAULT_WIDTH = 400; private static final int DEFAULT_HEIGHT = 400; 44 45 46 47 public void paintComponent(Graphics g) { var g2 = (Graphics2D) g; 48 49 // draw a rectangle 50 51 52 53 54 double double double double leftX = 100; topY = 100; width = 200; height = 150; 55 (Continues) From the Library of Hristo Dimov Hristov 586 Chapter 10 Graphical User Interface Programming Listing 10.3 (Continued) var rect = new Rectangle2D.Double(leftX, topY, width, height); g2.draw(rect); 56 57 58 // draw the enclosed ellipse 59 60 var ellipse = new Ellipse2D.Double(); ellipse.setFrame(rect); g2.draw(ellipse); 61 62 63 64 // draw a diagonal line 65 66 g2.draw(new Line2D.Double(leftX, topY, leftX + width, topY + height)); 67 68 // draw a circle with the same center 69 70 double centerX = rect.getCenterX(); double centerY = rect.getCenterY(); double radius = 150; 71 72 73 74 var circle = new Ellipse2D.Double(); circle.setFrameFromCenter(centerX, centerY, centerX + radius, centerY + radius); g2.draw(circle); 75 76 77 } 78 79 public Dimension getPreferredSize() { return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT); } 80 81 82 83 84 } java.awt.geom.RectangularShape 1.2 • • • • • • double double double double double double getCenterX() getCenterY() getMinX() getMinY() getMaxX() getMaxY() returns the center, minimum, or maximum x or y value of the enclosing rectangle. (Continues) From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 587 java.awt.geom.RectangularShape 1.2 (Continued) • double getWidth() • double getHeight() returns the width or height of the enclosing rectangle. • double getX() • double getY() returns the x or y coordinate of the top left corner of the enclosing rectangle. java.awt.geom.Rectangle2D.Double 1.2 • Rectangle2D.Double(double x, double y, double w, double h) constructs a rectangle with the given top left corner, width, and height. java.awt.geom.Ellipse2D.Double 1.2 • Ellipse2D.Double(double x, double y, double w, double h) constructs an ellipse whose bounding rectangle has the given top left corner, width, and height. java.awt.geom.Point2D.Double 1.2 • Point2D.Double(double x, double y) constructs a point with the given coordinates. java.awt.geom.Line2D.Double 1.2 • Line2D.Double(Point2D start, Point2D end) • Line2D.Double(double startX, double startY, double endX, double endY) constructs a line with the given start and end points. 10.3.2 Using Color The setPaint method of the Graphics2D class lets you select a color that is used for all subsequent drawing operations on the graphics context. For example: From the Library of Hristo Dimov Hristov 588 Chapter 10 Graphical User Interface Programming g2.setPaint(Color.RED); g2.drawString("Warning!", 100, 100); You can fill the interiors of closed shapes (such as rectangles or ellipses) with a color. Simply call fill instead of draw: Rectangle2D rect = . . .; g2.setPaint(Color.RED); g2.fill(rect); // fills rect with red To draw in multiple colors, select a color, draw or fill, then select another color, and draw or fill again. NOTE: The fill method paints one fewer pixel to the right and the bottom. For example, if you draw a new Rectangle2D.Double(0, 0, 10, 20), then the drawing includes the pixels with x = 10 and y = 20. If you fill the same rectangle, those pixels are not painted. Define colors with the Color class. The java.awt.Color class offers predefined constants for the following 13 standard colors: BLACK, BLUE, CYAN, DARK_GRAY, GRAY, GREEN, LIGHT_GRAY, MAGENTA, ORANGE, PINK, RED, WHITE, YELLOW You can specify a custom color by creating a Color object by its red, green, and blue components, each a value between 0 and 255: g2.setPaint(new Color(0, 128, 128)); // a dull blue-green g2.drawString("Welcome!", 75, 125); NOTE: In addition to solid colors, you can call setPaint with instances of classes that implement the Paint interface. This enables drawing with gradients and textures. To set the background color, use the setBackground method of the Component class, an ancestor of JComponent. var component = new MyComponent(); component.setBackground(Color.PINK); There is also a setForeground method. It specifies the default color that is used for drawing on the component. From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 589 java.awt.Color 1.0 • Color(int r, int g, int b) creates a color object with the given red, green, and blue components between 0 and 255. java.awt.Graphics2D 1.2 • Paint getPaint() • void setPaint(Paint p) gets or sets the paint property of this graphics context. The Color class implements the Paint interface. Therefore, you can use this method to set the paint attribute to a solid color. • void fill(Shape s) fills the shape with the current paint. java.awt.Component 1.0 • • • • Color getForeground() Color getBackground() void setForeground(Color c) void setBackground(Color c) gets or sets the foreground or background color. 10.3.3 Using Fonts The “Not a Hello World” program at the beginning of this chapter displayed a string in the default font. Sometimes, you will want to show your text in a different font. You can specify a font by its font face name. A font face name is composed of a font family name, such as “Helvetica”, and an optional suffix such as “Bold”. For example, the font faces “Helvetica” and “Helvetica Bold” are both considered to be part of the family named “Helvetica.” To find out which fonts are available on a particular computer, call the getAvailableFontFamilyNames method of the GraphicsEnvironment class. The method returns an array of strings containing the names of all available fonts. To obtain an instance of the GraphicsEnvironment class that describes the graphics environment of the user’s system, use the static getLocalGraphicsEnvironment method. The following program prints the names of all fonts on your system: From the Library of Hristo Dimov Hristov 590 Chapter 10 Graphical User Interface Programming import java.awt.*; public class ListFonts { public static void main(String[] args) { String[] fontNames = GraphicsEnvironment .getLocalGraphicsEnvironment() .getAvailableFontFamilyNames(); for (String fontName : fontNames) System.out.println(fontName); } } The AWT defines five logical font names: SansSerif Serif Monospaced Dialog DialogInput These names are always mapped to some fonts that actually exist on the client machine. For example, on a Windows system, SansSerif is mapped to Arial. In addition, the Oracle JDK always includes three font families named “Lucida Sans,” “Lucida Bright,” and “Lucida Sans Typewriter.” To draw characters in a font, you must first create an object of the class Font. Specify the font face name, the font style, and the point size. Here is an example of how you construct a Font object: var sansbold14 = new Font("SansSerif", Font.BOLD, 14); The third argument is the point size. Points are commonly used in typography to indicate the size of a font. There are 72 points per inch. You can use a logical font name in place of the font face name in the Font constructor. Specify the style (plain, bold, italic, or bold italic) by setting the second Font constructor argument to one of the following values: Font.PLAIN Font.BOLD Font.ITALIC Font.BOLD + Font.ITALIC From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 591 The font is plain with a font size of 1 point. Use the deriveFont method to get a font of the desired size: Font f = f1.deriveFont(14.0F); CAUTION: There are two overloaded versions of the deriveFont method. One of them (with a float parameter) sets the font size, the other (with an int parameter) sets the font style. Thus, f1.deriveFont(14) sets the style and not the size! (The result is an italic font because it happens that the binary representation of 14 has the ITALIC bit but not the BOLD bit set.) Here’s the code that displays the string “Hello, World!” in the standard sans serif font on your system, using 14-point bold type: var sansbold14 = new Font("SansSerif", Font.BOLD, 14); g2.setFont(sansbold14); var message = "Hello, World!"; g2.drawString(message, 75, 100); Next, let’s center the string in its component instead of drawing it at an arbitrary position. We need to know the width and height of the string in pixels. These dimensions depend on three factors: • The font used (in our case, sans serif, bold, 14 point); • The string (in our case, “Hello, World!”); and • The device on which the font is drawn (in our case, the user’s screen). To obtain an object that represents the font characteristics of the screen device, call the getFontRenderContext method of the Graphics2D class. It returns an object of the FontRenderContext class. Simply pass that object to the getStringBounds method of the Font class: FontRenderContext context = g2.getFontRenderContext(); Rectangle2D bounds = sansbold14.getStringBounds(message, context); The getStringBounds method returns a rectangle that encloses the string. To interpret the dimensions of that rectangle, you should know some basic typesetting terms (see Figure 10.9). The baseline is the imaginary line where, for example, the bottom of a character like ‘e’ rests. The ascent is the distance from the baseline to the top of an ascender, which is the upper part of a letter like ‘b’ or ‘k’, or an uppercase character. The descent is the distance from the baseline to a descender, which is the lower portion of a letter like ‘p’ or ‘g’. From the Library of Hristo Dimov Hristov 592 Chapter 10 Graphical User Interface Programming Figure 10.9 Typesetting terms illustrated Leading is the space between the descent of one line and the ascent of the next line. (The term has its origin from the strips of lead that typesetters used to separate lines.) The height of a font is the distance between successive baselines, which is the same as descent + leading + ascent. The width of the rectangle that the getStringBounds method returns is the horizontal extent of the string. The height of the rectangle is the sum of ascent, descent, and leading. The rectangle has its origin at the baseline of the string. The top y coordinate of the rectangle is negative. Thus, you can obtain string width, height, and ascent as follows: double stringWidth = bounds.getWidth(); double stringHeight = bounds.getHeight(); double ascent = -bounds.getY(); If you need to know the descent or leading, use the getLineMetrics method of the Font class. That method returns an object of the LineMetrics class, which has methods to obtain the descent and leading: LineMetrics metrics = f.getLineMetrics(message, context); float descent = metrics.getDescent(); float leading = metrics.getLeading(); NOTE: When you need to compute layout dimensions outside the paintComponent method, you can’t obtain the font render context from the Graphics2D object. Instead, call the getFontMetrics method of the JComponent class and then call getFontRenderContext. FontRenderContext context = getFontMetrics(f).getFontRenderContext(); To show that the positioning is accurate, the sample program in Listing 10.4 centers the string in the frame and draws the baseline and the bounding rectangle. Figure 10.10 shows the screen display. From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 593 Figure 10.10 Drawing the baseline and string bounds Listing 10.4 font/FontTest.java 1 package font; 2 3 4 5 6 import import import import java.awt.*; java.awt.font.*; java.awt.geom.*; javax.swing.*; 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 /** * @version 1.35 2018-04-10 * @author Cay Horstmann */ public class FontTest { public static void main(String[] args) { EventQueue.invokeLater(() -> { var frame = new FontFrame(); frame.setTitle("FontTest"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }); } } 25 26 27 28 29 30 31 32 /** * A frame with a text message component. */ class FontFrame extends JFrame { public FontFrame() { (Continues) From the Library of Hristo Dimov Hristov 594 Chapter 10 Graphical User Interface Programming Listing 10.4 (Continued) add(new FontComponent()); pack(); 33 34 } 35 36 } 37 38 39 40 41 42 43 44 /** * A component that shows a */ class FontComponent extends { private static final int private static final int centered message in a box. JComponent DEFAULT_WIDTH = 300; DEFAULT_HEIGHT = 200; 45 46 47 48 public void paintComponent(Graphics g) { var g2 = (Graphics2D) g; 49 50 var message = "Hello, World!"; 51 52 53 var f = new Font("Serif", Font.BOLD, 36); g2.setFont(f); 54 55 // measure the size of the message 56 57 58 FontRenderContext context = g2.getFontRenderContext(); Rectangle2D bounds = f.getStringBounds(message, context); 59 60 // set (x,y) = top left corner of text 61 62 63 double x = (getWidth() - bounds.getWidth()) / 2; double y = (getHeight() - bounds.getHeight()) / 2; 64 65 // add ascent to y to reach the baseline 66 67 68 double ascent = -bounds.getY(); double baseY = y + ascent; 69 70 // draw the message 71 72 g2.drawString(message, (int) x, (int) baseY); 73 74 g2.setPaint(Color.LIGHT_GRAY); 75 76 // draw the baseline 77 78 g2.draw(new Line2D.Double(x, baseY, x + bounds.getWidth(), baseY)); From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 595 79 // draw the enclosing rectangle 80 81 var rect = new Rectangle2D.Double(x, y, bounds.getWidth(), bounds.getHeight()); g2.draw(rect); 82 83 } 84 85 public Dimension getPreferredSize() { return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT); } 86 87 88 89 90 } java.awt.Font 1.0 • Font(String name, int style, int size) creates a new font object. The font name is either a font face name (such as "Helvetica Bold") or a logical font name (such as "Serif", "SansSerif"). The style is one of Font.PLAIN, Font.BOLD, Font.ITALIC, or Font.BOLD + Font.ITALIC. • String getFontName() gets the font face name (such as "Helvetica Bold"). • String getFamily() gets the font family name (such as "Helvetica"). • String getName() gets the logical name (such as "SansSerif") if the font was created with a logical font name; otherwise, gets the font face name. • Rectangle2D getStringBounds(String s, FontRenderContext context) 1.2 returns a rectangle that encloses the string. The origin of the rectangle falls on the baseline. The top y coordinate of the rectangle equals the negative of the ascent. The height of the rectangle equals the sum of ascent, descent, and leading. The width equals the string width. • LineMetrics getLineMetrics(String s, FontRenderContext context) 1.2 returns a line metrics object to determine the extent of the string. • Font deriveFont(int style) 1.2 • Font deriveFont(float size) 1.2 • Font deriveFont(int style, float size) 1.2 returns a new font that is equal to this font, except that it has the given size and style. From the Library of Hristo Dimov Hristov 596 Chapter 10 Graphical User Interface Programming java.awt.font.LineMetrics 1.2 • float getAscent() gets the font ascent—the distance from the baseline to the tops of uppercase characters. • float getDescent() gets the font descent—the distance from the baseline to the bottoms of descenders. • float getLeading() gets the font leading—the space between the bottom of one line of text and the top of the next line. • float getHeight() gets the total height of the font—the distance between the two baselines of text (descent + leading + ascent). java.awt.Graphics2D 1.2 • FontRenderContext getFontRenderContext() gets a font render context that specifies font characteristics in this graphics context. • void drawString(String str, float x, float y) draws a string in the current font and color. javax.swing.JComponent 1.2 • FontMetrics getFontMetrics(Font f) 5 gets the font metrics for the given font. The FontMetrics class is a precursor to the LineMetrics class. java.awt.FontMetrics 1.0 • FontRenderContext getFontRenderContext() 1.2 gets a font render context for the font. From the Library of Hristo Dimov Hristov 10.3 Displaying Information in a Component 597 10.3.4 Displaying Images You can use the ImageIcon class to read an image from a file: Image image = new ImageIcon(filename).getImage(); Now the variable image contains a reference to an object that encapsulates the image data. Display the image with the drawImage method of the Graphics class. public void paintComponent(Graphics g) { . . . g.drawImage(image, x, y, null); } We can take this a little bit further and tile the window with the graphics image. The result looks like the screen shown in Figure 10.11. We do the tiling in the paintComponent method. We first draw one copy of the image in the top left corner and then use the copyArea call to copy it into the entire window: for (int i = 0; i * imageWidth <= getWidth(); i++) for (int j = 0; j * imageHeight <= getHeight(); j++) if (i + j > 0) g.copyArea(0, 0, imageWidth, imageHeight, i * imageWidth, j * imageHeight); Figure 10.11 Window with tiled graphics image java.awt.Graphics 1.0 • boolean drawImage(Image img, int x, int y, ImageObserver observer) • boolean drawImage(Image img, int x, int y, int width, int height, ImageObserver observer) draws an unscaled or scaled image. Note: This call may return before the image is drawn. The imageObserver object is notified of the rendering progress. This was a useful feature in the distant past. Nowadays, just pass a null observer. • void copyArea(int x, int y, int width, int height, int dx, int dy) copies an area of the screen. The dx and dy parameters are the distance from the source area to the target area. From the Library of Hristo Dimov Hristov 598 Chapter 10 Graphical User Interface Programming 10.4 Event Handling Any operating environment that supports GUIs constantly monitors events such as keystrokes or mouse clicks. These events are then reported to the programs that are running. Each program then decides what, if anything, to do in response to these events. 10.4.1 Basic Event Handling Concepts In the Java AWT, event sources (such as buttons or scrollbars) have methods that allow you to register event listeners—objects that carry out the desired response to the event. When an event listener is notified about an event, information about the event is encapsulated in an event object. In Java, all event objects ultimately derive from the class java.util.EventObject. Of course, there are subclasses for each event type, such as ActionEvent and WindowEvent. Different event sources can produce different kinds of events. For example, a button can send ActionEvent objects, whereas a window can send WindowEvent objects. To sum up, here’s an overview of how event handling in the AWT works: • An event listener is an instance of a class that implements a listener interface. • An event source is an object that can register listener objects and send them event objects. • The event source sends out event objects to all registered listeners when that event occurs. • The listener objects then uses the information in the event object to determine their reaction to the event. Figure 10.12 shows the relationship between the event handling classes and interfaces. Here is an example for specifying a listener: ActionListener listener = . . .; var button = new JButton("OK"); button.addActionListener(listener); From the Library of Hristo Dimov Hristov 10.4 Event Handling 599 Figure 10.12 Relationship between event sources and listeners Now the listener object is notified whenever an “action event” occurs in the button. For buttons, as you might expect, an action event is a button click. To implement the ActionListener interface, the listener class must have a method called actionPerformed that receives an ActionEvent object as a parameter. class MyListener implements ActionListener { . . . public void actionPerformed(ActionEvent event) { // reaction to button click goes here . . . } } Whenever the user clicks the button, the JButton object creates an ActionEvent object and calls listener.actionPerformed(event), passing that event object. An event source such as a button can have multiple listeners. In that case, the button calls the actionPerformed methods of all listeners whenever the user clicks the button. Figure 10.13 shows the interaction between the event source, event listener, and event object. From the Library of Hristo Dimov Hristov 600 Chapter 10 Graphical User Interface Programming Figure 10.13 Event notification 10.4.2 Example: Handling a Button Click As a way of getting comfortable with the event delegation model, let’s work through all the details needed for the simple example of responding to a button click. For this example, we will show a panel populated with three buttons. Three listener objects are added as action listeners to the buttons. From the Library of Hristo Dimov Hristov 10.4 Event Handling 601 With this scenario, each time a user clicks on any of the buttons on the panel, the associated listener object receives an ActionEvent that indicates a button click. In our sample program, the listener object will then change the background color of the panel. Before we can show you the program that listens to button clicks, we first need to explain how to create buttons and how to add them to a panel. To create a button, specify a label string, an icon, or both in the button constructor. Here are two examples: var yellowButton = new JButton("Yellow"); var blueButton = new JButton(new ImageIcon("blue-ball.gif")); Call the add method to add the buttons to a panel: var yellowButton = new JButton("Yellow"); var blueButton = new JButton("Blue"); var redButton = new JButton("Red"); buttonPanel.add(yellowButton); buttonPanel.add(blueButton); buttonPanel.add(redButton); Figure 10.14 shows the result. Figure 10.14 A panel filled with buttons Next, we need to add code that listens to these buttons. This requires classes that implement the ActionListener interface, which, as we just mentioned, has one method: actionPerformed, whose signature looks like this: public void actionPerformed(ActionEvent event) From the Library of Hristo Dimov Hristov 602 Chapter 10 Graphical User Interface Programming The way to use the ActionListener interface is the same in all situations: The actionPerformed method (which is the only method in ActionListener) takes an object of type ActionEvent as a parameter. This event object gives you information about the event that happened. When a button is clicked, we want the background color of the panel to change to a particular color. We store the desired color in our listener class. class ColorAction implements ActionListener { private Color backgroundColor; public ColorAction(Color c) { backgroundColor = c; } public void actionPerformed(ActionEvent event) { // set panel background color . . . } } We then construct one object for each color and set the objects as the button listeners. var yellowAction = new ColorAction(Color.YELLOW); var blueAction = new ColorAction(Color.BLUE); var redAction = new ColorAction(Color.RED); yellowButton.addActionListener(yellowAction); blueButton.addActionListener(blueAction); redButton.addActionListener(redAction); For example, if a user clicks on the button marked “Yellow”, the actionPerformed method of the yellowAction object is called. Its backgroundColor instance field is set to Color.YELLOW, and it can now proceed to set the panel’s background color. Just one issue remains. The ColorAction object doesn’t have access to the buttonPanel variable. You can solve this problem in two ways. You can store the panel in the ColorAction object and set it in the ColorAction constructor. Or, more conveniently, you can make ColorAction into an inner class of the ButtonFrame class. Its methods can then access the outer panel automatically. Listing 10.5 contains the complete frame class. Whenever you click one of the buttons, the appropriate action listener changes the background color of the panel. From the Library of Hristo Dimov Hristov 10.4 Event Handling 603 Listing 10.5 button/ButtonFrame.java 1 package button; 2 3 4 5 import java.awt.*; import java.awt.event.*; import javax.swing.*; 6 7 8 9 10 11 12 13 14 /** * A frame with a button panel. */ public class ButtonFrame extends JFrame { private JPanel buttonPanel; private static final int DEFAULT_WIDTH = 300; private static final int DEFAULT_HEIGHT = 200; 15 16 17 18 public ButtonFrame() { setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT); 19 // create buttons var yellowButton = new JButton("Yellow"); var blueButton = new JButton("Blue"); var redButton = new JButton("Red"); 20 21 22 23 24 buttonPanel = new JPanel(); 25 26 // add buttons to panel buttonPanel.add(yellowButton); buttonPanel.add(blueButton); buttonPanel.add(redButton); 27 28 29 30 31 // add panel to frame add(buttonPanel); 32 33 34 // create button actions var yellowAction = new ColorAction(Color.YELLOW); var blueAction = new ColorAction(Color.BLUE); var redAction = new ColorAction(Color.RED); 35 36 37 38 39 // associate actions with buttons yellowButton.addActionListener(yellowAction); blueButton.addActionListener(blueAction); redButton.addActionListener(redAction); 40 41 42 43 44 } 45 (Continues) From the Library of Hristo Dimov Hristov 604 Chapter 10 Graphical User Interface Programming Listing 10.5 (Continued) /** * An action listener that sets the panel's background color. */ private class ColorAction implements ActionListener { private Color backgroundColor; 46 47 48 49 50 51 52 public ColorAction(Color c) { backgroundColor = c; } 53 54 55 56 57 public void actionPerformed(ActionEvent event) { buttonPanel.setBackground(backgroundColor); } 58 59 60 61 } 62 63 } javax.swing.JButton 1.2 • JButton(String label) • JButton(Icon icon) • JButton(String label, Icon icon) constructs a button. The label string can be plain text or HTML; for example, "<html>Ok</html>". java.awt.Container 1.0 • Component add(Component c) adds the component c to this container. 10.4.3 Specifying Listeners Concisely In the preceding section, we defined a class for the event listener and constructed three objects of that class. It is not all that common to have multiple instances of a listener class. Most commonly, each listener carries out a separate action. In that case, there is no need to make a separate class. Simply use a lambda expression: exitButton.addActionListener(event -> System.exit(0)); From the Library of Hristo Dimov Hristov 10.4 Event Handling 605 Now consider the case in which we have multiple related actions, such as the color buttons of the preceding section. In such a case, implement a helper method: public void makeButton(String name, Color backgroundColor) { var button = new JButton(name); buttonPanel.add(button); button.addActionListener(event -> buttonPanel.setBackground(backgroundColor)); } Note that the lambda expression refers to the parameter variable backgroundColor. Then we simply call makeButton("yellow", Color.YELLOW); makeButton("blue", Color.BLUE); makeButton("red", Color.RED); Here, we construct three listener objects, one for each color, without explicitly defining a class. Each time the helper method is called, it makes an instance of a class that implements the ActionListener interface. Its actionPerformed action references the backGroundColor value that is, in fact, stored with the listener object. However, all this happens without you having to explicitly define listener classes, instance variables, or constructors that set them. NOTE: In older code, you will often see the use of anonymous classes: exitButton.addActionListener(new ActionListener() { public void actionPerformed(new ActionEvent) { System.exit(0); } }); Of course, this rather verbose code is no longer necessary. Using a lambda expression is simpler and clearer. 10.4.4 Adapter Classes Not all events are as simple to handle as button clicks. Suppose you want to monitor when the user tries to close the main frame in order to put up a dialog and exit the program only when the user agrees. From the Library of Hristo Dimov Hristov 606 Chapter 10 Graphical User Interface Programming When the user tries to close a window, the JFrame object is the source of a WindowEvent. If you want to catch that event, you must have an appropriate listener object and add it to the frame’s list of window listeners. WindowListener listener = . . .; frame.addWindowListener(listener); The window listener must be an object of a class that implements the WindowListener interface. There are actually seven methods in the WindowListener interface. The frame calls them as the responses to seven distinct events that could happen to a window. The names are self-explanatory, except that “iconified” is usually called “minimized” under Windows. Here is the complete WindowListener interface: public interface WindowListener { void windowOpened(WindowEvent e); void windowClosing(WindowEvent e); void windowClosed(WindowEvent e); void windowIconified(WindowEvent e); void windowDeiconified(WindowEvent e); void windowActivated(WindowEvent e); void windowDeactivated(WindowEvent e); } Of course, we can define a class that implements the interface, add a call to System.exit(0) in the windowClosing method, and write do-nothing functions for the other six methods. However, typing code for six methods that don’t do anything is the kind of tedious busywork that nobody likes. To simplify this task, each of the AWT listener interfaces that have more than one method comes with a companion adapter class that implements all the methods in the interface but does nothing with them. For example, the WindowAdapter class has seven do-nothing methods. You extend the adapter class to specify the desired reactions to some, but not all, of the event types in the interface. (An interface such as ActionListener that has only a single method does not need an adapter class.) Here is how we can define a window listener that overrides the windowClosing method: class Terminator extends WindowAdapter { public void windowClosing(WindowEvent e) { if (user agrees) System.exit(0); } } From the Library of Hristo Dimov Hristov 10.4 Event Handling 607 Now you can register an object of type Terminator as the event listener: var listener = new Terminator(); frame.addWindowListener(listener); NOTE: Nowadays, one would implement do-nothing methods of the WindowListener interface as default methods. However, Swing was invented many years before there were default methods. java.awt.event.WindowListener 1.1 • void windowOpened(WindowEvent e) is called after the window has been opened. • void windowClosing(WindowEvent e) is called when the user has issued a window manager command to close the window. Note that the window will close only if its hide or dispose method is called. • void windowClosed(WindowEvent e) is called after the window has closed. • void windowIconified(WindowEvent e) is called after the window has been iconified. • void windowDeiconified(WindowEvent e) is called after the window has been deiconified. • void windowActivated(WindowEvent e) is called after the window has become active. Only a frame or dialog can be active. Typically, the window manager decorates the active window—for example, by highlighting the title bar. • void windowDeactivated(WindowEvent e) is called after the window has become deactivated. jav

Core Java Volume 1: Fundamentals, 11th Edition, Cay S. Horstmann

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